That'd be pretty cool. I like co-op play, but rarely is it that way for actual missions (single-player type play where you're going for a goal, not just completing the one mission). I'd also like to see a game where your actions in one mission reflect another mission, so if you kill a civilian in the second level, you won't lose, but you have to avoid the cops for the rest of the game. Or if you use up too much ammo, you run out towards the end of the game. It could be frustrating (having to start the whole thing over at the very end), but it'd add a lot more strategy to games. This probably wouldn't be good for UT2, but possibly for Deus Ex, even DNF.
Apple? Macintosh? What are these strange words you speak?