After using my iPhone 4 every single day for nearly a year and a half, the glass on the front and the back was still in near-mint condition. The only physical blemishes to the glass were some minor nicks that were only noticeable when all fingerprints had been wiped off and you were holding the phone at just the proper angle.
This is pretty much identical to the experience I’ve had with my iPhone 4, although I have managed to get one noticeable scratch on the back of the phone. Like Shawn, I’ve never used a case or a screen protector, but I’ve also never dropped it from a significant height onto a hard surface.