Sadly, Microsoft announced today that Halo 2's co-op mode will only be accessible through
offline splitscreen—no Live; no System Link. The news comes as a great disappointment to fans that had been
circulating the rumor of online co-op, even though neither Bungie nor Microsoft has ever hinted at such a
feature. However, 4-player splitscreen was confirmed for online multiplayer, providing some consolation for
the heartbroken. Splitscreen co-op in the original Halo was a great experience, but with the
sequel's revamped multiplayer, it's hard to believe that the feature-that-could-have-been will linger much longer
in our memories.
Engadget
TUAW