The PPC 970 are cheaper for Apple. And the G4 is near the end of it's life. It would seem logical that Apple give them G5s on the next revision but that might hurt Powermac sales. I say the revision after that.
That is if Apple wishes to keep Powerbooks affordable. Otherwise it would damage Powermac sales even if they wouldn't give the low end Powerbook a worse graphics card, worse drive, and other such things than the Powerbook and raised the price to $3000 CND alot of people would still be buying the Powerbook over the Powermac.
Or maybe I'm wrong and Apple will surprise us with a 1-1.2 GHZ Powerbook line for $3000 at Create. 😎