I noticed this on my Moms 14 inch iBook 1.33 SD:
The middle of the trackpad bows upward. The right and left edge stay put. Once bowed, if you press down on the center, it moves down flat again (with a slight crunchy noise) but then after a day or so it has bowed back up. It functions fine, and I have meant to bring it in to an Apple store to have it checked out, but havent gotten around to it. By comparison, my friends 12 inch iBook 1.33 does not have this issue at all.