Obama, for whatever reason, seems to play exceptionally well in the South all by himself. The unstated reason why you want to have a (white) southern (male) running mate is that they'll help you appeal to the red-neck vote, among other things. I'm not terribly confident that Obama's going to do well there, no matter who is running with him. On the other hand, if he can clean up with the Hispanic vote, that could well be enough to make the difference.