Apple is now voluntarily replacing the 3G with the 3GS if you have the defect and raise a stink about it.
Supposedly there is a known manufacturing defect that produces a hairline crack in the plastic on the back around the bottom of the dock connector.
It happens because the plastic is thinner on the back, and the normal wear of just using the dock connector causes flex, which causes this to crack.
Best bet is to request the replacement @ an Apple B&M store.
