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.
This require extends beyond digital copies of the games, and also includes the retail versions. On the plus side this integration allows global friends lists within steam, Cloud support for saved games and settings, leaderboards and achievements for both titles.
Napoleon Total War features a two player online campaign while Aliens vs. Predator features a four player online co-op survival mode pitting Space Marines vs Aliens.