I own a black iPod touch, because it looks cool :) plus, it was also the color of the very first iPod touch that I had, so I couldn't resist but with the variety of the 5th generation, maybe I'll get me a black and a sky blue one :)
Black, it is fine in all contexts of the use of the device overall. If you need the device to look extremely professional, sleek and cool overall, you are fine with black. White is a bit less formal and causes questions to be asked. Black is good all around.