Do you have any type of case on it? If so, the case can scratch it, I had an Agent18 on my previous gen and that caused 90% of the scratches on the back of mine. Is it white or black? Mine is white and I see 0 scratches, if it's black you're more likely to see scratches. Putting it in your pocket even with nothing else can cause scratches from the materials of your pocket, etc. I would recommend always placing the phone on the suede screen cleaner that came with the phone to protect it from scratches. I actually bought a full suede jacket for my iPhone 3G and am loving it so far. I have no scratches and I always have something soft to place my phone onto without worrying about it getting scratched. It also gives me the peace of mind when putting my phone into my pocket that I know it will not get scratched being in the suede jacket.
If interested it's by Waterfield and only 9.99 + shipping.