The first question I was asked last week when speaking to the Oneonta Macintosh Users Group was, "When is Leopard coming out?" After I finished explaining that I had no inside information and that the terms of my NDA would have prevented me from saying if I did, I revealed our internal speculation: October 26th, 2007.
When Apple missed its "first half of 2007" promised ship date for Leopard, reportedly due to shifting development resources to the iPhone, the date was reset for October (see "," 2007-04-16). Obviously, we're in October now, and if Apple remains true to form, they'll release on a Friday night. That narrows it to the 12th, 18th, or 26th. But which Friday?
With Panther, which shipped on 24-Oct-03 (the same day we launched Take Control Books with "Take Control of Upgrading to Panther" - see "," 2003-10-20), Apple foreshadowed the release roughly two weeks before with an announcement on 08-Oct-03. In 2005, Apple announced Tiger on 12-Apr-05, and roughly two weeks later shipped it on 29-Apr-05. If Apple were to announce Leopard this week, probably on the 9th or 10th, that leaves roughly two weeks before the 26th, fitting the pattern perfectly.
Of course, this all assumes that Leopard is ready to be released from its Cupertino cage. No one can comment on that without violating NDA, but it's important to remember that even after Apple declares a particular build the golden master and releases it to manufacturing (after which it must be distributed worldwide), Apple's programmers will be hard at work stamping out bugs that weren't considered show-stoppers. It's almost certain that 10.5.1 will appear within a few weeks of Leopard's eventual release. With Panther, 10.3.1 appeared on 10-Nov-03, a bit more than two weeks after the initial release. And Tiger's 10.4.1 appeared on 16-May-05, again, just slightly over two weeks after Tiger was unveiled to the public. So I think we can expect 10.5.1 in the middle of November, meaning that if you don't want to be on the bleeding edge, just wait a couple of weeks.
Apple could also change gears entirely - we were all expecting iLife '08 to appear at Macworld Expo SF in January 2007, and when it failed to materialize, the best guess was that it was tied to Leopard's release in some key way. Given the August 2007 release of iLife '08, that speculation didn't pan out. Plus, Apple has taken to invitation-only press events for unveiling new products, so it's conceivable that there's a postcard in the mail right now.
But my bet is still on October 26th, and if you want to get Leopard as quickly as possible without actually waiting in line at an Apple Store, the easiest approach is probably to pre-order either or  from Amazon.com. Oh, and Amazon's estimated ship date? October 31st. Spooky, eh?