I think people who are "half" black identify themselves as black, rather than white, because that's what they see in the mirror, and quite frankly, that's what everyone else sees.

If you saw Obama walking down the street and someone asked you for a descpription, you'd more than likely say, Tall, skinny black guy, NOT tall, skinny half-white guy.

By the way---GREAT point Lilo. Italian immigrants, such as all four of my grandparents, were hardly identified as white.