White - so people know you've got the 16GB and not the 8GB "poor person's" iPhone.
I wouldn't really say that's a "poor person's iPhone" until the back casing is blue
Seriously speaking, the color is really based on the owner's preference. Although on my case, it was
the first day of the release and my store ran out of the black ones so I had to get the white ones instead,
so far everything's good.