I've had several cases, and they've all scratched my iPhone. Scratched the back, scratched the bottom, scratched the bezel.. everywhere. However, I haven't noticed any damage done from my Incase Slider which has been on my 3GS for about a week and a half. That may be because it's white, but the bezel looks fine too.
Edit: I just took off my Incase to check, and while the iPhone still looks perfect, the inside of the case itself sure doesn't. Normal or not?