My White iPhone 3G 16 GB has had some very minute chips and cracks out of the box, right where the plastic meets the steel bezel. The cracks are between the sleep switch and the bezel, the volume switch and the bezel, between the screw holes on the bottom and the bezel and the headphone jack and the bezel. Radiating cracks have started to develop around the camera lens as well. The chips are at the top of the phone where the plastic mates with the bezel by the SIM slot. I just chalked it up to poor manufacturing tolerances and I even pointed it out to the Apple store salesperson when i bought it but more as a point of curiosity than wanting a replacement. As of right now, you have to look closely to see them and they haven't presented structural problems, yet. If they get worse I will set an appointment with the genius bar.
If I remember anything from my industrial technology college courses, it would seem that maybe the plastic cooled too rapidly near these openings in the plastic, causing these fractures. Just a guess.