Battery life on the 15" is pretty poor. This is a sad, but blatant fact. I get around 2.5 hours with "medium smart usage". iBooks kick the ass of PowerBooks in terms of battery life, as do Centrinos. I assume the 12" is appreciable better. Battery life really is the only thing that I am actively not happy about with my PowerBook.
AFAIK, third-Party manufacturers have only been able to increase the default battery capacity on the PB G4 15" by
11% compared to 42% on the last model.
You will need to take a power adapter/extra battery with you if you intend to work intensively away from home for over 2 hours.