Recent news came out for this game, too. You can read the full article here;
Bungie.net : Inside Bungie : News But in a nutshell it boils down to this;
FOUR PLAYER CO-OP, BITCHES!!! And the co-op this time will support both system link and xbox live. Oh god that is badass. I was really going to be disappointed if we couldn't at least be doing the two player co-ops over system link and xbox live and I wasn't too happy that Halo2 didn't support it. But not only did they finally get the god-given sense to toss that in, it's
four players! I creamed my damn pants when I read that, and I think I very well might have creamed again just now.