Can you list an FPS that has the ability to play against AI with a buddy of yours over the internet / lan? Not just a game with online deathmatch but actually fighting the CPU.. Sort of like Splinter Cell Co-Op.
I was going to say UT, but I have heard that Halo 2 was like that. I heard that you can have a mate or something to help you in campaign mode, but its a rumor at the moment, I can't find that option in my copy.
Its a very interesting concept though.