+1 to M$.
http://www.gamespot.com/news/6175937.html

http://www.gamespot.com/news/6175937.html
Bungie said:Today the studio confirmed a much-speculated piece of information about the campaign mode, stating on its Web site that it will indeed support online cooperative multiplayer for up to four players.
Since there's only one Master Chief, the four-player online campaign mode will include three more playable characters: Halo 2's Arbiter and two of his fellow Covenant Elites. All characters will play identically, but the Elites will wield carbines instead of battle rifles. That's an awful lot of firepower to throw into the same situations as the game's single-player campaign, but Bungie says it's addressing the various gameplay issues it creates.