This is interesting.

Nearly three weeks after Apple (AAPL) started shipping Leopard, the sixth version of its flagship operating system, and urged Mac users around the world to step up to OS X 10.5, it has just released the largest software update in memory for its previous two versions, Tiger and Panther.

The major improvement in OS 10.4.11, besides a slew of scary sounding security patches, is that it gives Tiger users Safari 3.0, the version of Apple's Web browser that was included in Leopard's $129 purchase price. But there are changes across the board, including upgrades in most of Apple's pro products. See the official list below the fold.

According to John Gruber, writing in Daring Fireball, this is the first time he can remember Apple delivering a point upgrade (i.e. to .11) to the previous Mac OS X generation after the release of a new one. As more than one commenter has remarked, it shows unusual concern in a personal computer company for its legacy users.

Software update OS X Version 10.4.11 is a monster, weighing in at 321.5 megs in its largest configuration (combo update for Intel Macs), a download that would take, as Tidbits points out, 18 hours over a dial-up modem or 3 minutes via a home fiber link.

Apple recommends that you backup your files before upgrading and warns that you "may experience unexpected results if you have third-party system software modifications installed, or if you've modified the operating system through other means."

But judging from early returns in the Apple.com discussion boards, the software update appears to be relatively problem-free. The main complaint — that the weather and stock widgets were broken — seems to stem from problems at Apple's end. Perhaps its server farms are busy delivering those 321.5 meg files.

You can get the upgrade through the usual Software Update function or by clicking on one of these links.

• Mac OS X 10.4.11 Combo Update (PPC) – 180.8MB

• Mac OS X 10.4.11 Update (Intel) – 128MB

• Mac OS X 10.4.11 Combo Update (Intel) – 321.5MB

• Mac OS X 10.4.11 Update (PPC) – 67.9MB

Below the fold, Apple's list of improvements:

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With three weeks left before the promised ship date of OS X Leopard, the long-awaited and much-delayed sixth major update of Apple's (AAPL) flagship Macintosh operating system, Piper Jaffray's Gene Munster is already calculating its impact on the company's revenue stream.

In a note to clients issued this morning, Munster observes that OS X Tiger, Leopard's predecessor, was also released at the end of the first month of a fiscal quarter (April 29, 2005 vs. Oct. 26, 2007). He writes:

At that time, the OS X installed base was 12 million and Tiger sales added $125 million to the quarter. The Mac OS X installed base is now approximately 23 million, so we expect Leopard to add approximately $240 million to the Dec. 2007 quarter. This assumes similar uptake rates to the Tiger launch, which saw 15% of the user base upgrade in just 6 weeks (eventually 66% of the user base upgraded to Tiger).

Looking ahead to the next Macworld, Steve Jobs' favorite venue for announcing new products, Munster anticipates one of two possiblities:

• a multi-touch PDA slightly larger than an iPhone (which some are calling the new Newton)
• an ultra-portable Mac that's smaller than the smallest MacBook (which AppleInsider has dubbed the ThinBook)

"If Apple launches a new product at MacWorld in January," Munster writes, "we believe it will likely fall into one of these two categories."