Yes, the new PowerBooks are based on a new revision of the G4 chip that can out-clock the old ones. The PowerBooks get it, since the G5 PowerBook is still "a ways off". The desktops may or may not get it. My speculation is that the eMac just got refreshed to use the last of the old G4 chips, and that Apple is working on replacing the iMac with a G5 model and won't bother with putting the newest G4 variant in it unless the G5 upgrade project hits snags and will be delayed.
I suspect that Apple would like to sell desktops that at least match, maybe beat their portables, so they are working overtime to sort out issues and proceed with including the G5 across the line.
Crikey