There's another factor, too. Most iBooks (except the newest ones) sport a G3, whereas the PBs use G4s which draw a bit more juice to run and cool.
My three favourite battery-savings tips:
- disable Airport until you need to get your email or check the web;
- drop your screen brightness way down to the second-to-last notch;
- don't run antivirus or software updates when you're unplugged.