I was asking the same question myself. As a matter of fact i just ordered my Powerbook 17" today I've been using desktop PC's since i was 7, thats about 10 years. After using my bros iBook i realised how **** PC is
About the G5 Powerbook: I asked around the forums and the guy at the shop, he said that it is very hard to fit the G5 into a laptop for these reasons:
1) Processor is quite big
2) If they do fit it in a laptop there will be no DUAL processor, and the G5 standard 1.6Ghz doesnt have a lot of performace difference compared to a g4 1.5 (thats what the guy said)
3) Eventhough they fit a g5 into an iMac theres ALOT and ALOT of cooling, and if you put cooling like this into a notebook it will eat ur battery in 30 min.
I guess it will take minimum a year to develop all these aspects, and even when they do the price will be very high. So if u get a PB g4 now u can still use it for 2 or 3 more years (even more prolly) and wait till the prices drop for the Powerbook g5 if that ever comes out.
Im getting my Powerbook G4 in 3 weeks, and i cant wait to convert