Sega's next two PC co-op titles will require Steam to play according to Shacknews.  Aliens vs. Predator, which releases on February 16th, and Napoleon Total War, which releases on February 23rd, both will require Steam to play any of the game's modes, including the cooperative ones.

Sega has just announced that the upcoming Turn Based and Real-Time strategy game, Napoleon Total War, will feature some new multiplayer campaign modes - including co-op play.  This mode of play was a result of the successful beta for Empire Total War which featured similar modes.