I think it's interesting that no one, no one has seriously pondered the possibility of OS X 10.6 (code named Fritz or whatever) being announced at WWDC. Thing is, that's where it SHOULD be announced. Looking at the schedule I think it's unlikely, but still possible. Only a few "to be announced" sessions, and mostly in the latter part of the week, so my expectations are low.
OS X is on an 18-24 month revision cycle. 10.5 was announced August 2006. It was supposed to ship May/June 2007 but slipped to October 2007 (26 months). So in theory 10.6 should be released anywhere in the December 2008-October 2009 range depending on how you look at it. Since we've heard nothing thus far, I'll bet it would be towards the latter side.
Developers need a WWDC during the development phase of an OS to learn the innards. We will never (or should never) have an OS release otherwise.
I think it's conceivable we'll start to hear official word of 10.6 at WWDC, but again I don't think we'll hear much. Certainly no seed distribution, etc.
Good timing might be for a formal announcement at MacWorld 2009, with shipping in September-November 2009. I think WWDC should be earlier in the cycle (assuming a semi-traditional June WWDC) but that could work.
I was really hoping for a 10.6 show at WWDC this year, but again it doesn't look promising. Maybe we'll get some vague mentions of new API's we'll see "in a future release" without any demo.
But Apple better already be working on it (should have started a year ago, with work in earnest starting last fall).
Another possibility, I suppose, is that the iPhone 2.0 codebase is closely related to the upcoming 10.6 codebase. Maybe that's how the 10.6 item got thrown in there.
Fritz, yeah, Fritz.
My complete speculation is that 10.5 is the last release that will support PPC. I think Apple is moving quickly to orphan these legacy platforms. Dropping all support for Classic mode and artificially limiting Leopard to a certain class of G4 systems and above are big indicators that future releases will not include PPC.
I think we'll have one more release for PPC, although I suspect there will be Intel-only features. I also think we'll have lots of Intel-only software in the meantime (including the Pro apps from Apple, and more from Adobe) to push people to Intel.
Microsoft got lots of negative feedback for requiring upgrades for Vista. I don't think Apple will risk the same. Intel MacPro is 22 months old now, same with XServe. I can't see Apple requiring someone with a three-year-old high-end Mac to upgrade their hardware. I could see them requiring a G5, but not Intel-only.