The iBooks have the older, MPC 7455 G4 with 256 KB level 2 cache.
The PowerBooks use the newer MPC 7447 G4 with 512 KB level 2 cache and other enhancements. That's why they're faster than the new iBooks.
It's a moot point now, but the IBM 750 GX G3 with 1MB level 2 cache at 1.1 GHz on 200 MHZ FSB might very well smoke the G4's. That was the chip that was rumored to go into the new iBooks. So much for rumor.