If you were counting on Leopard gaining an advantage from shipping before Vista, it looks like you're out of luck. Vista is in its last test release before shipment, and historically, the final release candidate becomes the supported RTM (ready to market) product. OEMs and volume licensees are expecting November delivery of the finished Vista, and the onesie twosie, shrink-wrap buyers will see Vista early next year. Subscribers to MSDN (Microsoft Developer Network, Microsoft's counterpart to the paid flavor of Apple Developer Connection) will probably get the RTM code sometime between those two dates.
I'm not being too cynical, am I, to suppose that despite all the long faces and equivocal mumbles by MS execs when asked about Vista availability, November was never really in doubt? I mean, it is nice to have your bases covered; no one would have been surprised or disappointed if Vista's initial deliveries to VIPs slipped into '07. But Microsoft has been telegraphing impending availability to partners, developers and even end-buyers with marketing campaigns that talk up Vista as though it's already gone platinum. It might as well have. Microsoft has pressurized the pipeline by teasing buyers and developers with seemingly uncertain availability. The media's shameless mockery of Microsoft for having blown Longhorn/Vista targets time and again has helped generate pent-up interest in Vista by turning mere delivery of the OS into page one news. (watch the headlines; the word "finally" will work its way into a lot of story titles)
To the dismay of so many observers, when Vista goes RTM, it'll explode. It'll shatter every standing record for software units and revenue in a given period of time. It'll bury everything else in a mudslide of press coverage, and it'll even make story #4 or #5 in non-technical publications and network news. I predict a BusinessWeek cover.
One could say that Vista's got a greased track to record sales: PC models that currently ship with Windows XP will begin shipping with Vista. Perhaps Microsoft will offer an install-time choice between OSes the way it initially did with Windows XP and Windows 2000 Professional. In any case, every PC with Vista pre-installed, even if unused, counts as a sale. I'm not suggesting there's anything dishonest about this. I'm just preparing you for mind-boggling Vista numbers in case you might find such statistics disturbing.
So the next few months will be all about Vista. Where, oh where does this leave Apple? When Vista reviews start running, we'll see the professional and lay media pile on with the predictions of doom for OS X and the "too little, too late" Leopard that journalists have never seen outside apple.com. Vista is it, and Macs are at a disadvantage for not shipping with Vista. Journalists of that ilk will null the value of OS X and insist that the true cost of a Mac is the machine plus the copy of Vista that really makes it work. There will be plenty of reasons for the Mac faithful to sound a call to arms.