I seriously doubt it. If it were a 64-bit OS, then it would HAVE to be backwards compatible with the G4 processors. It would be ridiculous for Apple to phase out the G4's this quick -- I think that OS X is fine in its current incarnation, and with a little tweaking here and there, it can run just fine on anything.
Maybe Apple will implement some 64-bit functions, but I doubt it. I'm not as educated as I'd like to be in the world of 32- and 64-bit OSs, but I think it would be a stupid move for Apple to alienate their biggest customer base.
I would be quite suprized if the core of OS X isn't already 64-bit clean. This means that it just has to be recompiled for a 64-bit processor and it will be fine. I would think that for 10.2 they will have all of the OS 64-bit clean and possibly they will have 2 versions of it, one for 32-bit processors (G3, G4) and one for the 64-bit (G5).
It should also be noted that the G5 can run as either 64 or 32-bit so they might just run the G5 at 32-bit for a while until there is enough units out there for them to re-compile the client version. One would hope they would recompile the server version for 64-bit as soon as possible, since the benefits of having a 64-bit OS are much more aparent with servers (much larger files, more ram, etc).
I can say with COMPLETE certainty that Apple will not release OS X 10.2 as a 64-bit OS that is not compatible with 32-bit chips. Why? Because Apple hasn't even released complete 64-bit processors! Also, Apple is still using G3 chips in their consumer line, and is probably going to get some improved ones that go to 1 GHz from IBM! Why in the WORLD would Apple alienate their consumer market from OS X when they are touting the ease of use of OS X? Simple logic, guys.
And, if G5s DO "come out" early next year, I'll bet it'll be a similar thing to what Jobs did at MWSF '01... he'll announce them shipping in a month or two. Something like that.
And Admiral, www.maccentral.com would only report on facts, not rumors. You probably heard it on www.mosr.com who quoted that one The Register article about the G5. But we ALL know how outrageously optimistic www.mosr.com is. At least I hope we all do, because MacOSRumors jumps on the gun far too soon far too many of the times.
And yes, the G5 will be a full 64-bit processor, from what I've read. The G4 isn't a 64-bit processor, it just has improvements over the G3 and a 128-bit velocity engine... that doesn't mean the whole chip is 64-bit.
even if 10.2 was a 64-bit os (highly unlikely I think), it could probably run on both 32 and 64-bit chips without modification, thanks to osx's next heritage.. the bundle format allowed (not sure if apple took it out for osx) multiple architectures to be supported with one 'fat' binary. with next, it was sparc, 68k, and i386.. Apple could simply use this to support two different ppc archs - ppc32 and ppc64 or whatever they'd be called.