when you carry anything with a screen in a backpack, you leave it open to being pushed on - not just by other things in the backpack like your wallet, water bottle, camera, etc, but by the bench you put it down on if anything gets pushed against it, by other people, other books, other backbacks, jackets or other things being put on top of it, etc.
i'd go with a hard case - yes, made for something else, but something that fits the cybook - before i'd carry mine in a backpack. but then i'm paranoid
