In case you missed it...
http://www.theregister.co.uk/2004/08...ores_detailed/
Sure looks like there will be the possibility of at least 3 more upgrade paths for G4 powerbooks:
1) scale the current G4 to ~2GHz
2) Build a dual core G4 processor
3) Scale it to beyond 2GHz
This sure gives Apple and IBM plenty of time to get their low-power G5 CPUs ready.