I have a zagg invisishield, prior to that I carried my N900 bare in my jeans and with the CP-321 case from noka, it got scratches both times.
Anyways, in the end it had tons of scratches so I replaced the screen layer and put the invisishield (but not before getting three (incredibly superficial) scratches).

In other words, get a screen protector. It is the only protection the N900 needs, it is built like a tank everywhere else.