What Obama's mixed background does represent is the old racist nightmare of miscegenation, especially as we're talking about a white woman and black man. I believe that a majority of white Americans considered mixed sexual relations as wrong well into the seventies.
Here in Colorado Springs, with its large military popluation, there are a lot of mixed race people with African and Asian heritages. They aren't labeled as "black." There are also quite a few mixed race families with Latino and European roots, and while they have dark skin they aren't "blacks" either. Nor are Indians or Arabs over here. I also know some very dark skinned African-Americans who speak with the received accent and have WASP values.
The U.S. military is pretty integrated, which has changed the viewpoint in that community. Not perfectly, though.
There is still some overt racism; we had a blatant example in my neighborhood just a few months ago. But I think it was an outlier...
