A few days ago an article for an upcoming EGM article on Halo 3 was leaked. In the article pieces about new weapons, vehicles, and such was shown, but one of the most significant pieces of the article was the part regarding online co-op for the game.
According Bungie’s Frank O’Connor the addition of online coop might not make it to the final product due to the single player level layouts. The levels are so vast that it would be possible for each player to be miles apart from one another at certain points in the game which would cause a major problem. They have ways of restricting this problem, but it would affect the gameplay in a way that would effectively ruin the game’s experience.
So, while it could happen, chances are all online coop modes, not just the rumored 4 person online coop, will not be included in the shipped product. Rather it’s more likely that a patch at some point after the release will add this particular mode if it’s released at all.