Since IBM seems to be capable of pushing out new chips quicker then Motorola, I expect that Apple will probably do more speed bumps with little fanfare over time.
Keep in mind WWDC this year was delayed, it might move back to April/Mayish next year. I do expect the first hardware changes in the G5 to probably show up around summer next year though. Most likely minor things, like bumping all models to PCI-X if it becomes cost effective enough.
Mind you, this is all speculation based on what I see, and no actual basis in fact. Pinning down specifics is pretty hard from the outside. And the info I do know from time to time, well, stays quiet.