this is a big worry among iBook users. i dont work at Apple, but seeing as IBM makes the newer G3 chips (700mhz and up) and are using a somewhat re-vamped architecture for them, i see Apple carrying out the life of the G3 before it would upgrade to a G4. They will probably get to 1GHz, then come up with a new style for the iBook and put the current G4 powerbook chip in it and upgrade the powerbook to a G5. these are only my predictions of course
My guess is a little different. I have an iBook 500Mhz and a 500Mhz Ti powerbook. The powerbook get much hotter. I don't know the limits of the G3. But it may be better reach the limit, because the hardware would have to change much. If the iBook moves to the G4, there would be some major changes made. At least IMO. I think the G3 still have some useful life left. It will be very interesting to see how Apple will change their product line.
the only problem i see there, is that Motorola made the G4, where as IBM makes the G3 & G5. As odd as it sounds i still think Apple would update the G3, unless IBM re-vamps the G4. Motorola hasn't made any real updates to the G4 since the iMac 1Ghz.