only a 'bit' cooler?

have you actually compared powerconsomption of the 7447a/b with the 970FX? It's comparable to the PM vs P4.
And regarding the different processors, they are different now as well, as long as they share the same PPC guidelines there should be no problem. It's a plain fact mobile processors are a different category. Both Intel and AMD use different chips for their mobile solutions as well. It would be pretty silly for Apple to use the same chip for both markets. That would mean either less performance on the desktop or more inches for the notebook.
To get back on the subject I expect a minor update to the powerbook together with a major update for the ibook to keep both apart.
12" pb around 1499
1.8 GHz 200MHz bus 7448 G4
ATI 9700 64 MB
DL superdrive standard
15" pb around 1999 / 2249
1.8 / 2.0 GHz 200 Mhz bus 7448 G4
ATI 9700 128 MB standard
DL superdrive standard
17" pb << around 2249 / 2499 cheaper 17" option perhaps?
1.8 / 2.0 GHz 200 Mhz bus 7448 G4
same gpu + vram
DL superdrive standard