I think the big winner will be President Obama! Think about it, right now his party is in power in both houses of congress during a horrible economic situation. He can't really blame congress too much because short of a filibuster, the republicans can't really do anything themselves.
If the congress changes hands, he gets someone to blame in 2012 for all the economic problems we have. He can rightfully say that they had power and did not want to work with him. Let's face it, things are going to get worse before they get better. Having someone to blame who has some power to fix a problem is far better than having bad luck or only yourself to blame.