Thanks for the responses. Given that the fighter we've had is a little TOO sticky for the DM's liking, I think we'll keep the combat challenge separate from the opportunity attacks. It's more complicated, but I don't like the idea of a fighter being able to attack 4 enemies who try to shift away from him in one round or try to attack the wizard next to him. Apprecitate the input.
I don't believe it is an Aoo either. I believe they made it an immediate action so that it would not be much more powerful than the Paladin Divine Challenge benefit. Divine Challenge only allows its damage once per round. Thus Combat Challenge only allows one immediate action attack per round. That makes Divine Chalenge and Combat Challenge balanced against each other, with Combat Challenge being slightly better.