If you go by a video on youtube where they put an iPhone in a bag with keys and shook the bag, well the screen should be pretty scratch proof.
If we go by real life experiences, the screen can and has been scratched. There are numerous reports on the Apple forums and other forums where people pulled the phone out of their pocket to discover a scratch.
Just be careful with it and put it in your pocket with nothing else in there and you should be OK. Mine is six months old and always stays in it's own pocket everyday. No scratches on mine.