Not necessarily, but the PowerBook was built out of a titanium alloy and the iBook some kind of polycarbonate. I'm no materials engineer but I can see an argum

Not necessarily, but the PowerBook was built out of a titanium alloy and the iBook some kind of polycarbonate. I'm no materials engineer but I can see an argument for the iBook needing to be thicker to support the same design loads. (I'm sure it was just a side benefit that it let them save a few bucks on display size, without having to also shrink the keyboard to an unusable size.)