While a two-player co-op mode has been confirmed for Halo 3 (player two assumes the role of the Arbiter) it looks like it'll be available in LAN or split-screen only as Bungie struggles to get the mode running over Xbox Live.
In the latest issues of EGM, it's reported that online co-op isn't happening, yet. "We're not dumb," says Bungie's Frank O'Connor. "We know that people want it and we're trying to make it happen. I think the biggest problem for us for online co-op is that we have a situation where you can be in a Warthog with five troops, almost a mile away from the other player. That's a significant challenge. And there's lots of design things you could do to prevent that from happening, but they would make it not feel like Halo anymore. If we can make it happen in a way that works well, we will - and if it works badly, we won't."
Bungie didn't rule out that online co-op could become a possibility via downloadable content post release. Looks like those rumours of four-player Live co-op have gone forever.
The same issue of EGM also revealed tons of new info on vehicles, equipment and weapons previously kept under wraps. New gear includes the Flare (a flashbang) a dual wieldable Brute shotgun dubbed the Mauler, a Gravity Hammer (send vehicles flying, apparently) and a Regenerator (opposite of the power drainer). New Vehicles to look out for include the Prowler, Hornet and Elephant, described as a Jawa Sandcrawler. A remake of multiplayer map Lockout was also revealed called Guardian.
The mag also hints that Marathon will be on Xbox Live Arcade by the time the mag goes on sale, which is August 11.