The problem with the natural selection arguments (dark skin makes it easier to hide) is that the drow's hair color should also be dark. It is (obviously) not.
The same holds for the "curse of blackness" theory- why would a race of elves be cursed with black skin and silver/white hair? While darkness may be associated with evil, I have never heard of contrasting hair color being associated with evil!
I have a theory of my own, but I need to google some references first...
[edit]
Ok. Look here:
http://abcnews.go.com/sections/us/DailyNews/bluecandidate_021003.html
The man has argyria, caused by drinking colloidal silver. The condition is permanent.
My theory is that the drow are colored the way they are because of minerals in their diet; probably adamantine. The have a mutation which protects them against slow adamantine poisoning by precipitating tiny particles of the stuff in (or under) their skin, making it dark.
Normal melanocytes are killed by the (slightly toxic) adamantine, so the skin of a drow elf has no protection against the sun; they burn easily. No melanin means that their hair is white, too.
Other creatures typically do not live long enough for adamantine poisoning to be a factor (elves are unusually long lived, and so have a longer time to be exposed to it), have a diet which minimizes adamantine exposure, or deal with the issue in another way.
Now suppose that racially drow are also pre-disposed to have dark skins with plentiful melanocytes. It is just that adamantine poisoning kills these melanocytes by the time they are a century old. But their genetic disposition means that drow (or half-drow) born and raised outside of the underdark (and not exposed to adamantine) still have dark skins, but with a different hue than the purplish-grey of their kin. Even in the underdark, young drow would be a more "natural" chocolaty brown or ebony, and not the weird color of mature drow. Though they could be given colloidal suspensions of adamantine to hasten the approach of their mature coloration.
In this theory, drow not exposed to adamantine will have dark hair, and skin that is a different shade from normal drow. They should also be no more sensitive to sunlight than someone whose ancestors are from the tropics.