Apple's (AAPL) fiscal third quarter earnings are due out Tuesday, July 21, and once again the Street is focused on the big numbers — revenues, earnings and units sold for the Mac, iPhone and iPod.

But savvy analysts will be paying closer attention to the number that is the best measure of a firm's profitibilty: gross margin, expressed as the ratio of profits to revenues. Or

(Revenue – Cost of sales) / Revenue

Apple's gross margins, which have averaged 34.8% over the past eight quarters, are the envy of the industry. Dell's (DELL) first quarter GM, by contrast, was 17.6% and the company warned Wall Street last week that it is expecting a "modest decline" next quarter.

In its April earnings call, Apple low-balled its guidance numbers as usual, forecasting a sharp drop in gross margins over the next 6 months. Specifically, it warned analysts to expect no better than 33% in Q3 and "about 30%" in Q4.

But Turley Muller, for one, doesn't buy those numbers, and he should know.

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Apple is set to release its second-quarter earnings on Wednesday, and by coincidence its shares closed on Friday at just over $161 — almost exactly where they stood three months earlier, before Apple's first-quarter earnings report.

Although the company in January posted the best earnings in its 32-year history, the Q1 report is remembered by investors as a disaster. In the weeks that followed, Apple (AAPL) shares fell more than 40 points — from above $160 to below $120 — knocking $36.5 billion off the company's market capitalization. Recession fears were a big factor in what turned out to be a three-month bungee jump, but what really spooked the market was Apple's Q2 earnings guidance: 94 cents per share, nearly 15% below the Street's average estimate of $1.09. [Reader "Mick" points out that hedge funds dumping Apple to prop up their shaky financial positions played a major role in the sell-off. He notes that institutions held 71% of Apple's shares before the plunge and 68% after.]

So there are two things to watch for on Wednesday: 1) Apple's sales figures for Q2, which should be stellar, and 2) what kind of guidance it gives for Q3, which is anybody's guess.

All signs point to an excellent second quarter for Apple. The consensus of analysts surveyed Monday was looking for the company to earn $1.07 a share on $6.95 billion in sales, versus the company's guidance of $0.94 on $6.8 billion

Strong sales of MacBooks led the quarter. IDC last week reported that, although growth in overall PC sales in the United States slowed last quarter to just 3%, Apple's computer shipments were up 25.1%. Gartner, using slightly different methodology, reported Mac sales up 32.5%.

If Apple's worldwide performance is anything like its domestic record, the company should easily beat the Street's consensus of 1.95 million Macs sold in the quarter. Piper Jaffray's Gene Munster is looking for Mac sales of 2-2.1 million; JP Morgan's Mark Moskowitz expects them to come in even higher, at 2.11 million. Either number would represent a near doubling of sales in just two years, as Ars Technica's handy bar graph shows.

The iPod picture is not quite as rosy. There is sure to be sharp seasonal falloff from the Christmas quarter, when Apple shipped 22.1 million units. JP Morgan's Moskowitz estimates that Apple sold 9.68 million iPods in Q2; Piper Jaffray's Munster is calling for somewhere between 10 to 10.5 million, reflecting a sales spurt late in the quarter sparked by a sharp price cut on the low-end iPod shuffle. According to Munster, the Street has already decided that the iPod's days of growth are behind it, and that the consensus is looking for sales of just under 53 million iPods in 2008 — essentially unchanged from 2007. Munster's more optimistic; he believes the iPod will evolve over the next 12 months from a stand-alone music player into a mobile Internet device that fits in your pocket, and he's looking for iPod sales to grow 10% year over year.

iPhone sales are harder to predict, given the spot shortages in the United States, excess inventory in Europe, and a chaotic black market in jailbroken iPhones in Asia and the developing world. Analysts' estimates are all over the lot. Moskowitz and Munster (to pick on those two one more time) differ by half a million units. Moskowitz expects Apple to report sales of 1.5 million iPhones; Munster is looking for 1.6 to 2 million. Charles Jade at Ars Technica's Infinite Loop speculates that the release date of the 3G iPhone may hinge on what the actual number turns out to be. He writes:

With a prediction of 10 million iPhones sold in CY 2008 … Apple must sell, on average, 2.5 million iPhones per quarter. … If the iPhone sold less than 2 million units this quarter, expect a 3G iPhone sooner rather than later. Conversely, if the current shortages are a result of insatiable lust for the greatest phone ever made, expect Apple to milk that cow for all it's worth before introducing a new model. (link)

When it comes to pricing Apple's shares, however, Wall Street cares less about the past than the future. The guidance Apple gave last October hinting at a blowout Christmas surprised analysts and help drive the stock to a record $200 a share in December. Although Apple beat everybody's expectations for the quarter, by the time the first quarter results came out, traders were focused on Q2. And when Apple shocked analysts in January with surprisingly pessimistic guidance, it triggered a 40 point fall.

Investors, some of whom lost millions in the debacle, were furious, and Apple was besieged by angry threats and e-mails. ("Straight out, bald face, criminal lying," was how one described Apple's Q2 guidance). Few expect the company to respond such complaints by sweetening its numbers; if anything, it is more likely to offer no guidance at all, especially for a quarter that is so hard to call. Although investors can look forward to a new iPhone and software developers kit in June, back-to-school sales in late summer, and Christmas sales before the end of the year, none of those expectations will show up in Q3 earnings.

If Apple does offers Q3 numbers, they are sure to be, as always, conservative. Apple, more than most companies, likes to make only promises it knows it can keep. But despite recent complaints, the fact is that its results do tend to track its guidance. The spreadsheet at left, produced by a member of TMO's Apple Finance Board who calls himself "awcabot," shows guidance and results quarter by quarter since 2002. Past performance is no guarantee, but over that time, revenues have exceeded guidance fairly dependably by an average of 5.7% and earnings by an average of 43.8%.

Take all this for what it's worth. Apple is a volatile stock, and it's especially volatile before and after earnings reports. We may not be in for another bungee jump, but for the next few days it could be a bumpy ride.

In terms of relations with its users and business partners, the quarter that ended Sept. 30 wasn't a particularly easy one for Apple (AAPL). There were complaints and lawsuits about the cost of replacing the iPhone's battery, the surprise $200 price cut, and the software update that "bricked" untold numbers of unlocked phones. Negotiations with Hollywood and the European iPhone partners were awkward and protracted and in the case of NBC ended in an impasse. Consumers who bought the new entry-level skinny iMacs are still complaining about low-quality screens and an unresolved software glitch that's causing random freezes.

But in terms of sales and profitability, the consensus among analysts is that the fiscal fourth quarter was a terrific quarter for Apple, buoyed by back-to-school specials, switchers fleeing Vista, and a flood of hot new products. The line on Apple heard most often on the Street — in a cliche borrowed from the automotive industry — is that Steve Jobs' profit-generating machine is firing on all cylinders these days and will handily beat Apple's guidance numbers (never particularly hard to do, given how conservative those numbers tend to be).

As the chart below the fold shows, the analysts who follow Apple most closely are marching pretty much in lockstep this quarter, with Cupertino's guidance as the sole outlier. Everybody seems to be expecting the company to report sales of more than 2 million Macs, 10 million iPods and a million iPhones, give or take a few truckloads.

The biggest news, says PiperJaffray's Gene Munster, is likely to be the disclosure for the first time of how much revenue Apple has been collecting — and amortizing over 24 months — from AT&T (T) for iPhone sales and monthly user fees. The precise terms of Apple's revenue-sharing arrangements with cellular carriers here and abroad have been, until now, a closely held secret. Munster estimates that the AT&T deal could add $10.6 million to Apple's bottom line in Q4 alone.

Apple will announce its quarterly earnings on Monday Oct. 22 after the market closes and meet by phone with reporters and analysts an hour later. Click here at 5 p.m. ET (2 p.m. PT) to listen in on Apple's webcast. Fortune's analysis of the results will be posted here.

Below, as promised, is a sampling of Q4 estimates:

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