This would have to be with a new, more efficient G4 (10 hour battery life???). If this rumor is true, which I find doubtful, it would also mean that Apple is now taking the strategy of not worrying about cannibalizing sales, and is pegging its fortunes increasingly to the success of the iBook. Again, though, this doesn't seem quite right.
Or maybe Apple's pressure on Motorola pushed them to develop under wraps something like a G4-M, a low-power G4 chip (could the 7457 really have 1/2 the power consumption of the existing G4?). Maybe Moto wanted to ensure that the G4 is used in the iBook and PowerBook for at least a couple more years (maybe to make back investments they've already made in R&D) before they bow out of the CPU market entirely....