Mike Isaac, All Things D:

A number of Apple early adopters claim that the back case of the iPhone 5 is highly susceptible to scratching and more visible signs of wear and tear, according to multiple reports from across the Web.
Perhaps even more serious, numerous accounts state that users are receiving iPhone 5 devices damaged directly out of the packaging, before customers have had chance to actually lay hands the device.
It seems to be an issue with the anodized aluminum surface, and the apparent difficulty that comes with keeping the device from getting nicks and scrapes. It appears that the black version is either more susceptible to damage than its white counterpart, or signs of visible wear are more readily visible due to the higher contrast in colors.