I just got a new Macbook, and having the experience that most any mac keyboard leaves nasty marks on the screen I decided to get me an isaver (
http://www.i-stuff.de).
At first I was a little annoyed that it doesn't cover the trackpad, since that can put marks on the screen too. What's worse, however, is that the darn thing leaves its own frikkin' marks, where the isaver folds! Well, just thought I'd warn you. I guess I'll go back to using a piece of paper. Suits me right for getting a piece of cloth for 15 bucks.