During a official press conference in Amsterdam Microsoft and Bungie confirmed that Halo 3 will feature 4 player cooperative play on Xbox Live. More info coming soon.
"In Halo 3, the third installment of the legendary epic, the war has come to a climax; the Flood continues to grow in numbers and strength, the armies of Earth are scattered and decimated, and the Covenant threatens to activate the Halo, effectively destroying all life in the galaxy. Master Chief's mission is to stop the Covenant, destroy the impending Flood threat and, ultimately, save mankind."
Worldwide availability starting september 25, 2007
4 player co-op is regarding the singleplayer missions.