If it was proven that Obama wasn't legally eligible to be POTUS then what?

I am not looking for an argument on if he is or not, just legally what would happen, would all the things he signed be revoked or would they be allowed to stand, would Biden be allowed to step in or would he be discounted because of Obama winning the election under fraud, would McCain and Palin win by defualt and be handed the presidency? Like I said no sides on this I think it would be interesting to know how this would be handled if it ever happened, It would differ from anything else because it would void his election and not just be an impeachment or death like what has happened before.