It could be part of the model because it's part of the business logic of the game.
It could be part of the controller because it could be seen as simulating player input, which would be considered part of the controller, right? Or would it?
What about a normal enemy, like a goomba in Mario?
UPDATE: Wow, that's really not the answer I was expecting. As far as I could tell, A.I. is an internal part of the autonomous game system, hence model. I'm still not convinced.