I can't escape the notion that a meaningful slice of voters are telling pollsters they support Obama...while harboring their own private doubts. Nobody wants to appear "racist", y'know. These voters are probably spread throughout the demographic spectrum (including blacks).
What they might do in the privacy of the voting booth -- when a decision is finally forced upon them -- will be the opposite of their polling response in many cases.
In North Berkeley (CA) today: only a few weeks away from the election (I just received my sample ballot today) and I think I saw maybe only two or three BO bumper stickers, and no BO yard or window signs at all.