It might be a bit hard to find willing participants to do that test. Then again, I can only give anecdotal comments. I've had a N900 in my backbag, mixed with all kinds of stuff, keys and all, for over 6 months now. (They're free for us, in a way, so we don't really pay good care to them, they're abused a lot.) My screen has about 5-6 tiny hairline scratches. With the screen on it's virtually impossible to see them, with the screen off and looking against a light reflection, yes you can spot them.

So I wouldn't be overly worried with screen scratches. But then again, as people say, if you are worried, buy a screen protector. Yes, and if you take your car keys and scratch the screen violently, I'm sure you can leave a mark there.