I've only had my ibook for a matter of weeks. Maybe a month. In the time that I have owned it, I never closed the lid without putting down the foam screen protector that it came with. Somehow despite that, I opened it up today and noticed keyboard marks on the screen. Now I think when I had my last iBook (clamshell), I never used the protector, and I don't recall ever getting keyboard marks.
Anyway, I don't really care as long as I can just clean them off periodically. I can do that, right? At least now I don't feel the need to bother with that foam thing every time I open up the laptop.