The most likely update, if/when it happens, is a switch to the 7448 G4 processor. This is not a dual (or dual-core) processor, but it's a good upgrade from the current Powerbook processor, the 7447A. The 7448 is faster (over 10 percent) and requires about half as much power as the 7447A. I'd also look for an upgrade in the 12" RAM to 512, and hopefully new graphics cards (or more RAM in the current graphics cards) all down the line. Those are the obvious components, and the simplest, so I think they're likely, if the update happens at all.
For you, I would say that the upgrade won't make too much of a difference. There have been rumors (or wishful thinking) of a widescreen 13" replacing the 12", but I think that sort of form-factor change is unlikely to happen before the switch to Intel (if only because it would require significant resources which are currently focused on the Intel transition).
At this point, I want to wait a little longer for the speed and battery boosts that will hopefully come with the update. Are they major upgrades? No. But it will be nice. Then again, it's even nicer to actually have a computer and get to use it, rather than just read rumors and ponder the machinations of the infinite loop (
1 Infinite Loop, that is). I think you'll end up happy either way.