How about 500 and 600 Mhz G4 iBooks?
Remember when Apple introduced the Tibooks the two models were 400 and 500Mhz G4's upgraded from 400 and 500Mhz G3 Pismo Powerbooks. It would be just like the Powerbook transition so that way Apple can keep the G4 iBooks at 100Mhz bus and hopefully the iBooks will stay cool and quiet like the G3 iBooks of the present.
I think that would be the safest transition to the G4 for the iBook so it wont interfere with Powerbook sales.
Also Apple should introduce Bluetooth in the G4 iBook with a better video card and a audio input to go along with
iMusic a basic digital audio recording software with a built in software synthesizer and MIDI capabilities.
I wont go into any more details because this is going off the topic.
Markus