Adding to the growing buzz — some of which he helped create – around a possible refresh of Apple's (AAPL) iMac line of desktop computers, Kaufman Bros.' Shaw Wu issued a report to clients Monday morning with the latest update from his supply chain sources.

Noting that AppleInsider reported Friday that Apple seems to be running short of iMacs, Wu makes the following bullet points:

• Timing: "In our experience, when AAPL sends an advisory to its channel partners of limited availability and inventory of existing models are drawn down, it is highly likely that a product refresh is within a few weeks."
• Specs: Wu is now hearing that the new iMacs will be available in both dual-core and quad-core models. "We believe this makes sense as this helps AAPL create better tiers within the iMac family, utilizing quad-core for the high-end, and dual-core for mid-range and low-end."
• Market share: Wu estimates that the iMac accounted for 25% of Apple's Mac business last quarter (portables accounted for more than 70%) but could account for as much as 36% after a refresh.
• Mac Pro: He believes Apple's high-end tower, while less important than the iMac, is also likely to get updated. "We think a refresh utilizing upcoming Intel 'Nehalem' 8-core processors (and with two enabling a 16-core) would bring it better price performance and help jump start this highly profitable segment."

Wu leaves himself considerable wiggle room in terms of timing. Even as he writes about an update within a few weeks, he quotes sources indicating that the iMac is "due for a refresh in the March or June quarters."

The March quarter wraps up in eight weeks. The end of the June quarter is 20 weeks away.

See also: Why the iMac is late

Where was the new iMac that Apple watchers expected Steve Jobs — or rather, Phil Schiller — to unveil at Macworld?

In a report to clients issued Monday, Kaufman Bros.' analyst Shaw Wu says it will be out before March, or June at the latest, and he offers three reasons that the refresh of Apple's best-selling desktop machine is running behind schedule. According to his latest supply chain checks:

1. Apple hasn't yet decided whether to power the new iMac with Intel quad-core processors or newer dual-core processors with larger caches. "While quad-core would provide a material improvement in performance and potentially  jump start sales," he writes, "it could cannibalize the Mac Pro, its high-end tower."
2. Apple computers tend to run hot, and the iMacs vents and cooling systems may need to be redesigned, he says, "to deal with higher heat dissipation."
3. The timing of Snow Leopard — the next-generation Mac OS X announced by Steve Jobs last June but still without a firm release date — could be an issue. "While Leopard would take advantage of multiple cores," he writes, "Snow Leopard takes it to the next level with better support for multi-core, multi-processors, and OpenCL, with enhanced graphics capability."

Wu points out what last week's earnings report made abundantly clear: Apple's desktop machines are due for an overhaul. The company's desktop business was down 25% year-to-year in the quarter that ended in December. Only boffo sales of the MacBook and MacBook Pro — up 34% — saved its Mac numbers from going south last quarter.

Wu is sticking with his Apple (AAPL) price target of $120 a share, which as he points out is 11 times free cash flow earnings for the calendar 2009, by his estimate, but only 7.5X if you exclude Apple's enviable $28.1 billion cash hoard.

None of this is set in stone — especially as long as Steve Jobs retains the prerogative to change his mind at the last minute — but AppleInsider has posted the most definitive road map to date of Apple's (AAPL) fall product lineup.

Citing unnamed "people familiar with the situation," AppleInsider's Kasper Jade ticks off a schedule of release for a batch of new iPods, overhauled notebooks and refreshed iMacs, confirming several rumors that have been floating around for weeks and adding a few of his own.

Taken alone, none of these announcements sound quite big enough to account for the sharp drop in the company's gross margins — from 34.1% to 31.5% — CFO Peter Oppenheimer warned analysts to expect this quarter, citing a mysterious "future product transition." But together, they might do the trick.

Here they are, in the order Jade expects them to be released:

• New iPods in September. Digg founder Kevin Rose, an Apple watcher with a track record considerably more checkered than Jade's, predicted last Friday that Apple would soon revamp its entire iPod line, cutting prices sharply, making cosmetic changes to the iPod touch and introducing a significantly redesigned iPod nano with a long thin screen (link). Over the weekend Rose specified a date on which all this would occur: September 9 (link). Without endorsing that particular Tuesday as the date, Jade's sources confirm that Steve Jobs himself will headline a special event tentatively scheduled for the second week of September in which "cheaper and slightly modified iPod touches players and new iPod nanos and related service announcements are expected to take center stage." (link) One thing that will not be announced at that event, according to Jade's sources, is the long-rumored Newton-like handheld multi-touch device.
• New MacBooks and MacBook Pros in September or October. Jade's sources are cagey about the timing here, but they were explicit in saying that new versions of Apple's hot-selling notebook computers would not be available until some time after the new iPods are introduced. Whether that means they are announced at a separate event — perhaps in October — or whether they will be announced at the same event and shipped some weeks later is one of those mysteries that may not get cleared up until the event actually occurs. Among the changes expected: a MacBook clad in aluminum (like the Pro) rather than plastic; tapering around the edges (a la MacBook Air) to produce a slimming effect; a mysterious new chipset (but still with an Intel (INTC) CPU); and a newly designed battery cover and latch that offer easier access to the hard drive.
• Refreshed 20-inch and 24-inch iMacs in November. This is a brand new rumor, rather than a rumor confirming old rumors. According to Jade:
  • "People familiar with these plans have described the refresh to consist of 'speed bumps' rather than major internal or external changes. Based on the roadmap presented to AppleInsider, these systems would debut later this fall following the release of the new MacBooks, making their way to market with little fanfare."

Not expected before the end of the year are refreshes of the Mac Pro or the Mac mini, although Jade's sources report that the latter, once given up for dead, is getting a "major overhaul — the most significant in the mini's short history."

[Timeline and photos courtesy of AppleInsider.]

MacRumors has issued an update of its immensely useful Buyer's Guide — a consumer-oriented cheat sheet that tracks the update cycle of Apple's product line and offers informed opinions about whether you should go ahead buy that MacBook Pro you've been lusting after or wait for the next model. As MacRumors put it:

Apple updates their products in a very consistent manner. A Mac comes out at a certain price with certain features. The price and features of that particular Mac stay exactly the same throughout the lifespan of the product. So, if a customer buys on Day #1, they are getting the fastest/newest technology for the dollar. The problem, however, is that 8 months later, on the day prior to its refresh, that Mac costs the exact same money, but contains 8 month old technology. (link)

Although based on rumors and second-hand reports, the Guide is pretty dependable, especially since Apple (AAPL) switched to Intel chips. Intel (INTC) is quite open about its product plans, and Apple tends to switch to their newest processors in a fairly predictable timeframe. (Although as MacRumors notes, Intel's switch to the Nehalem microarchitecture, due late this year, could stretch out some Apple product cycles.)

To see the full 2008-2009 Buyer's Guide, click here. This is a summary of their recommendations:

• iPod classic: Buy only if you need it – Approaching the end of a cycle
• iPod touch: Neutral – Mid product cycle
• iPod nano: Buy only if you need it – Approaching the end of a cycle
• iPod shuffle: Buy – Product recently updated
• Mac mini: Don't Buy – Updates soon
• Mac Pro: Neutral – Mid product cycle
• MacBook: Buy – Product recently updated
• MacBook Pro: Buy – Product recently updated
• iPhone: Buy only if you need it – Approaching the end of a cycle
• LCDs: Don't Buy – Updates soon
• Xserve: Buy – Product recently updated

There's lots more information in the full Buyer's Guide, including historical release dates, days since update and links to recent news.

One caveat: you take a risk when you buy a computer on Day #1, as MacRumors suggests. You might want to monitor Apple's discussion boards for few weeks to see what problems emerge. Let the company and the users who like to live on the bleeding edge work out the kinks before you buy.

What is it about Apple computers that makes them run so hot?

Complaints about overheating notebooks — Apple doesn't call them laptops anymore for reasons that become obvious once you use them for a few minutes — surfaced soon after the release of both the MacBook Pro in early 2006 and the MacBook later that spring.

Now the problem is the new MacBook Air. Despite assurances from Apple (AAPL) reps at MacWorld that it ran cooler than its bulkier predecessors, Apple's discussion boards are filled with messages about fans running a full throttle, machines overheating and occasional lock-ups. As of Thursday afternoon, the topic "MacBook Air Overheating" had been viewed 3,135 times and a second topic, "MacBook Air intermittent freezing problem," had drawn 2,541 views.

"I spoke to Apple and the first thing I was advised to do was to remove all cables and the battery!" one user reported. Whoever was manning that help line apparently was not aware that the battery on the MacBook Air can't be removed.

On Monday, Apple issued a software update — MacBook Air SMC Update 1.0 — that "fine tunes the speed and operation of the internal fan," but reports of its effectiveness are mixed. One user said his machine had been relatively quiet since the upgrade. Another said it "didn't work at all." Others reported that when they tried to install it they were told that their MacBook Air was already up to date.

The broader problem, one suspects, is that Apple has once again put design considerations ahead of performance and pushed the new machine's heat tolerance just a bit too far.

The Macworld Conference & Expo, Silicon Valley's largest technology trade show, opens Monday. But the moment everyone is waiting for comes Tuesday morning, when Steve Jobs makes his annual keynote address at San Francisco's Moscone Center.

Jobs has set a high bar for himself. At Macworld 2006, he introduced the first Intel (INTC)-based Macs — sparking a burst of sales that nearly doubled Apple's (AAPL) market share from roughly 4% to something approaching 8% (link). At Macworld 2007 he unveiled not just the all-but-forgotten Apple TV, but also the iPhone — a device that in nearly everybody's book turned out to be the machine of the year.

What can Jobs do to top that?

There's no shortage of speculation. The Apple rumor machinery has grown so elaborate that for the second year in a row, Ars Technica's John Siracusa has published a keynote Bingo card (available in PDF format here and in iPhone format here), with boxes to be filled in as Jobs makes his announcements, introduces his guests and trots out his trademark rhetorical flourishes. (The rules of the game are spelled out here.)

Nobody has yet shouted out "Bingo!" in middle of a Steve Jobs presentation — a moment brilliantly anticipated in IBM's buzzword Bingo TV ad (link) — but this could be the year.

Some of Siracusa's boxes are obviously more important than others. A couple (Mac Pro and Xserve) were preemptively filled last week, and there are a few key possibilities that he missed. Watch especially for:

• A Skinny MacBook. Probably the leading candidate for Jobs' one-more-thing moment, it's already been named — Macbook air, thin, nano and mini — and imagined in PhotoShop (see here, for example) by bloggers who should know better. Likely specs: 12 to 13-inch. LED backlit screen, under 3 lbs., half as thick as today's MacBooks, 32, 64 or even 128GB solid-state flash drive, priced around $1,600.
• iPhone updates. A bump in capacity from 8GB to 16GB and maybe 32GB is expected, as well as a preview of the software developers toolkit (SDK) promised for February; we might even get a few demos from developers, like EA, who were seeded with the SDK last fall. A 3G iPhone and a Newton-type tablet are reported to be in the works, but not yet ready for prime time.
• Movie rentals. This is the item Hollywood is following most closely. It's been widely reported that Fox and Disney are likely to make movies available on iTunes for overnight rental (at $3 to $5 for 24 hours) or for purchase for roughly the price of a shrink-wrapped DVD. If, as rumored, Paramount, Lions Gate and Warner Bros join them, the flood of fresh video content could breath new life into the Apple TV. (The Associated Press reported Sunday that Netflix (NFLX), anticipating such a move by Apple, will offer unlimited monthly video streaming.)
• DRM-free Music. Having famously championed the cause with his February 2007 Thoughts on Music memo, it would be surprising — and disappointing — if Jobs did not use this opportunity to announce a significant expansion of the DRM-free offerings in the iTunes Store, especially after the last of the major labels announced last week that they were putting their music on Amazon.com (AMZN) without copy protection.
• Microsoft (MSFT) Office 2008. No surprises here, since the reviews are already in, but an excuse for what should be the most lavish after-hours party of the show.
• The Beatles. It's about time. Just in case, Yoko Ono's John Lennon Educational Tour Bus mobile recording studio is making the trip from its Las Vegas unveiling at the Consumer Electronics Show to be at Macworld. A few hours after Jobs' speech, there's a press reception in the bus that's co-sponsored by Apple.

You already see the flashbulbs popping, right? But is it enough? Apple's marketing machinery is like a shark that must keep swimming or die. Even if nearly every square on the Bingo card were to be filled on Tuesday, would Jobs have delivered the kind of innovation and buzz the faithful have come to expect?

And then there's Wall Street to consider. Apple was the high-flying tech stock of year, its share prices having more than doubled in 2007. But as a CNNMoney headline put it on Friday, "What've you done for me lately?" The stock fell nearly 30 points over the last two weeks, which could be taken as a measure of traders' uncertaintly. (Or it could just be a well-timed pause to set up the Macworld effect, the short-term bump tech share prices often enjoy after a Steve Jobs' keynote.)

No matter how high the bar, Jupiter Research analyst Michael Gartenberg is confident that Jobs will clear it. "This is a company that thinks in terms of strategy," he says. "Do I think they'll deliver something as disruptive as the iPhone? No. You don't achieve that kind of disruption every week; it would be tantamount to getting into a whole new industry. But somehow Jobs always manages to meet expectations, even if the expectations are different."

To find out how different, tune in Tuesday for Fortune senior writer Jon Fortt live blogging from the keynote at fortune.com/bigtech, video coverage from CNNMoney.com and our post-keynote analysis here on Tuesday afternoon.

Like the Oscars for the technical categories, which the Academy of Motion Picture Arts and Sciences hands out ahead of time rather than bog down the major Academy Award presentations, Apple (AAPL) is using this week to introduce new products that Steve Jobs feels aren't worthy of his keynote at Macworld next week.

Today it announced two of them:

• A new Mac Pro, with two 2.8 GHz Quad-Core Intel Xeon processors, a 320GB drive, and up to 4 terabytes of internal storage for photo editors, designers and other memory hogs (starting price: $2,799). See here for press release.
• A new Xserve with a standard single 64-bit 2.8 GHz Quad-Core Xeon processor, a 80 GB drive, up to 3 terabytes of storage and an unlimited client license for Mac OS X Server version 10.5 Leopard (starting price: $2,999). See here for press release.