And some other shit.
From Bungie.net

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It’s time for us to sack up and tell you what’s what regarding online co-op.

So you were probably wondering why we didn’t want to commit to two player co-op online over Xbox Live. We certainly got plenty of mail asking, no, demanding that we make it happen. Of course we were working on it, but we were also working on something better. Not two player co-op. Not three-player co-op, but up to four player co-op. Online. On Xbox Live, or sure, System Link if you prefer.

That is correct – up to four player co-op in Campaign mode on Xbox Live or System Link.

We’ve been playing it for quite some time, and bluntly, having some of our greatest ever Halo experiences. We wanted to make sure all our “t”s were crossed and our “i”s dotted before unveiling this important new part of Halo 3’s Campaign mode.

Apart from the horrifying network challenges this new mode presented us, were the various gameplay balance issues it adds to the fray. So we got it all together, polished it, and that’s why we’re revealing it now. As a result, you’re going to want to play co-op as hard as you can stomach it. We’d certainly recommend Legendary, but we’ll be revealing some more ways to make the game more interesting and challenging for multiple players in the very near future.

Acting as a team is a fantastic new twist to the Campaign gameplay – something that works as well on foot, in close confines as it does on the battlefield, with large scale vehicle mayhem. Scared of Jackal snipers? Send out a scout to see what dangers lie ahead. Terrified of an open field? Flank your enemies and swipe their rides. The combinations and scenarios are endless – and you’ll be able to enjoy them time and time again in Saved Films of your co-op exploits.

We mentioned that we’ve been having a blast. We have a lot of stories. Many of them would require that we explain some of the reasons replay will be a big factor, but not quite yet. Soon, we promise.

Like every other aspect of Halo 3, this mode is chosen from a Lobby. Simply select Campaign, choose your network (Xbox Live or System Link) and invite your friends to join you, or they can simply join your session once it’s set up and in a lobby (but they cannot join a game once it’s in-progress). They will pop into your lobby instantly, and when you have all the players you need (anywhere from one to four) you can begin the game from the beginning – or from any of your saved checkpoints.

Split-screen co-op is as ever, limited to two players per screen – but they can if they wish join two other friends online or via System Link.

For fictional reasons, we decided to avoid Halo’s slightly surreal compromise of two identical Master Chiefs and rather, use this opportunity to expand the Halo universe just a little bit. Player one will control the Master Chief, player two will control the Arbiter and players three and four will get the chance to control two brand new Elite characters.


http://www.bungie.net/News/content....=thesoundofsack
^ more about characters, too