I have heard of people cracking the screen when they put it in the airplane seat in front of them and their knee hit it -- so I think localized pressure directly on the screen isn't a great idea.
That being said, I've dropped my PRS-500 more times than I'd have liked, and the worst part it ever got was the odd nick on the dark blue metal. Never had a scratch on the screen in the two years I owned it. I now have a PRS-505 that's dropped face down onto tile from a propped up position. Wasn't thrilled about it but nary a scratch. I think they're pretty sturdy -- I still try my best to handle mine with kid gloves though
On a side note, there was a goofy Sony Reader Revolution guy at the SonyStyle store I went to and he dropped his PRS-505 from waist high, hard. He also tossed it like a frisbee against a wall there, and generally roughhoused it to no ill effect. (Aside from making me cringe when he did it) He also says he's left it on the top of his car and driven off and in Vegas he dropped it from a ten story building.....but those sound like tall tales ;-)