I've had an iSkin on an iBook for three years now. It does make marks on the screen -- people have called them "scuff" marks, but actually, at least in my case, they come off. Interestingly (this is a *matte* screen), a Mr. Clean Magic Eraser is actually perfectly safe and very effective at getting these off the screen. I lived with the marks on the screen for a while because other methods didn't work to get them off, and then finally, I lost patience and tried this, and it worked. The screen looks as good as new. I'm not sure what would happen on a glossy screen, though.
If you want to be really careful, then there is always the option to take the iSkin out and/or use one of those pieces of cloth that lay in between the screen and the keyboard to protect the screen.
