Former CNET alum Tim Moynihan performed extensive stress testing on the Apple iPad for PC World, and came to two conclusions: the iPad is fragile, but the screen is virtually scratch-proof.
Buying a case for the iPad makes a lot of sense, but adhesive or film screen protectors are a waste of money. We know the iPad's screen gets smudgey and the glass is fragile, but sticking something on top of it isn't going to help.
There are exceptions, however. Screen films that offer privacy when viewed at off-angles, or prevent glare, may be useful for particular applications. For most users, we strongly advise against mucking up the screen with films and adhesives. A good case is all the protection you need.