you'll never see a dual G3. If I remember correctly, the G3 just doesn't do MP very well. not to mention it would drain the battery faster and create issues with heat probably (the G3 may run pretty cool, but throw two processors in a small enclosure and it at least bears apple thinking about it).
I don't think you'll see any processor in between the G3 and G4.
The G3 already is an IBM processor.
speed bumps most likely, but I think you might see a G4 in there soon enough. Maybe not right away, but it wouldn't really surprised me to see them in there. After all, apple can do enough with the 12 inch powerbook to differentiate it by a few hundred dollars and still make it attractive. They can do things like built in bluetooth, airport extreme, better optical drives, bigger hard drives, more ram, etc, etc that would cost extra or be unavailable on the iBooks to make up the difference. I mean, really you could just think of the 12 inch powerbook as the new incarnation of the iBook special edition. I don't see it encessarily as a barrier to a G4 iBook, but it will probably take a little while longer.