Its amazing how clueless so many of you are. Apple needs marketshare. And Apple needs this marketshare to continue rising. One of the easiest ways they can determine this marketshare is by total volume sales of major OS releases. Leopard has been incredibly successful for Apple. "Snow Leopard" needs to be even more successful from a sales and marketing standpoint to prove that they are definitely continuing to move forward on the marketshare front. But if they drop PPC support this soon from the release of the last G4 notebooks and G5 desktops, that is a significant portion of their installed base who will not be buying the new OS. And I can guarantee you that the vast majority of people who are left behind because of a totally arbitrary switch to Intel are not going to just go out and immediately buy a new computer (and who says that this would be an Apple computer even if it they did; I know that if they dropped PPC support in 10.6, that would probably be enough for me to look into "other" options of running 10.6 so that I could most economically replace my dropped system). Yes, maybe those left behind will eventually purchase a new mac, but it certainly won't happen in a mob-like immediate upon-release movement, enough so that Apple would be able to market Snow Leopard as "the fastest and best selling Mac OS ever" (including new hw sales). Believe it or not, Apple still needs these types of marketing opportunities and press releases.
Of all the rumors I have read regarding 10.6 and machine support, the only one that I could half see really happening is a move to 64bit-only machines. This would still leave a significant number of users in the cold, but certainly wouldn't be as bad as no PPC support at all. Afterall, the G5's are still very capable machines. I have four macs running Leopard today: Dual G4 1.25GHz, C2D MBP 2.16GHz, Quad G5 2.5GHZ, and XServe Octal 3.0GHz. I find the G4 runs Leopard pretty well, as long as you have a decent GPU (I have a 7800GS in it). From a performance standpoint, my MBP certainly beats the G4 hands-down, but then again my MBP is pretty much trounced by my Quad G5. And of course the XServe trounces the G5... But as far as "running the OS", all four of these systems run Leopard very well, its the applications that primarily separate the four.
If 10.6 is all about performance tuning, then I could easily (unfortunately) see the G4 being left behind, but I can't see the G5's being left in the dust this soon simply because of its relative performance to even today's currently shipping macs and due to how many professional users they will be leaving behind (pros who use macs for their business are the most common G5 tower owner, and for Apple to abandon those machines now would be a slap in-the-face considering all the years pros were practically the only ones keeping Apple in business). The counter argument that these G5s will still be usable after a switch to an Intel-only OS is laughable. As soon as Apple drops PPC support, so will third-parties. The big developers are only going to follow Apple's lead on this one, and while I agree that this is going to have to happen eventually, I can't fathom it happening before 2010. Plus, even if there are no enhancements or tuning updates whatsoever for PPC in 10.6 and the PPC support is the same as it was in 10.5, this would still be a whole lot better than "officially" dropping PPC with this release.