Both my wife and I have a black 3G each since last Christmas, and both of us use siclicone (back only cover) cases. On the front, we both use the screen protector. I can almost get away with selling them brand new!!!!! Believe it or not! They are both in imaculate condition, with barely a smudge on them.
I don't want to forget to mention that I work in the construction field, Hardwood flooring installer, and finnisher.
So it all depends where you keep it, whats in your pocket along with it, and wether your case is any good....thats if you use one at all.
A friend of mine has a white one, without a case, and he just leaves it anywhere... and it looks like it as gone to hell and back. It hurst me just to watch him handle it!!!!!!