I was at the Apple Store in Soho the other day, and I decided to take the time to look over all of the 3GS's they had out on display. (The Geniuses kept on asking if I had any questions

)
Most of them had no major defects as far as I could tell. Out of the twenty some odd 3GS's I checked, I'd say about five of them had major hairline cracks on the bottom. A couple of them were chipped/scratched, but that was probably just be careless customers dropping them onto the counter.
Most of them had fairly even bezels, but two of them had noticeably uneven bezels. The right side of both of them were way higher than they should have been and when I slid my finger across the screen, it would always get caught on the bezel.
Overall, not too noticeable. If anything, just swap your iPhone out for a new one, but do not leave the store. Check your iPhone thoroughly, and if there are any major defects, immediately tell the Genius that is helping you out. Same goes for when you buy the iPhone.
On an interesting side note, when I was at the Apple Store, there was a lady trying to use the keyboard on the iPhone 3GS, and she was claiming that the keyboard was terrible and wouldn't type whatever she was entering.
That's cause she had 1 1/2 inch long nails and was using her entire finger onto the keys. The Genius that was helping her was trying to tell her that she needed to use the tips of her fingers to type, and that her fingernails were making that a problem. Then the lady proceeded to curse him out, call him stupid, call the iPhone a POS and storm away.
Poor Genius...he was just trying to help.