Look at the two girls in the picture. Would you say that one is black and the other white? Would you change your answer if you learned that they are twins?
If you want to read up on the story behind this, see the snopes.com entry on them. This newspaper story has some information on the science involved, but it actually gets several things wrong, particularly in calaculating the probabilities. This Mixed Media Watch post and the ensuing discussion had some good analysis. There are more pictures in all three places, including a picture with both parents.
I'm seriously interested in what people think of these girls, not because of some morbid fascination with classifying people but because people's instinctive reactions to this are directly related to my dissertation. Are people instinctively inclined to treat one girl as black and the other as white (rather than both as black, both as white, neither as black or white, or whatever other combinations might be possible)? Is your answer one thing if you don't know they're related and another if you find out that they're twins?