I currently have a black iPad but also had a white iPhone 4 which I sold and here's the reason:
The white devices themselves are beautiful. There's no doubt about that, especially the iPhone. BUT here are the two issues I couldn't get over. The white bezel looks painted on and raised. By raised, I mean you can tell the bezel and glass is above the actual display. This was especially evident with brightness lowered. It just didn't feel right. I always felt like the bezel was supposed to be black and that the white was painted on. Number two- the white bezel is almost transluscent around the areas closest to the display. You can see a gray ghosting around the display perimeter. Also, the white is not a crisp pure white. Like someone already mentioned, if your pages are white, it simply accentuates that the white border is a bit off white- especially in incandescent lighting.
Anyway, there's my opinion- the black is a much more cohesive device and just feels right. The white always feels like a novelty of some sort and you will never forget that it is white- the bezel becomes a feature of attention and you cannot forget it subconsciously.