If you've played online co-op for Gears of War, it's basically the same thing for Halo 3. The differences are that only two players can split-screen, and the other two have to be other Xbox Live players. They can drop in at anytime if the party isn't full.

As promised, Bungie's Luke Smith has revealed the details of Halo 3's surprising new mode: a four-player cooperative online adventure that lets you and up to three buddies take on the game's Campaign mode as a group. Calling the mode "some of our greatest ever Halo experiences," Smith gave the following examples of how the cooperative gameplay can affect the game:

While only two of these players will be able to share a single screen, you'll be able to add a third and fourth either via local System Link mode or online through Xbox Live. And you won't all have to play as Master Chief -- the other three players will be totally different characters, as shown in this screen released today. Each character may have a different starting weapon, but will otherwise be identical.

While there is no drop-in gameplay, you can start a multiplayer game from any of your previous saved checkpoints in the single-player mode.

Co-op in Halo 3[Bungie]

Supernouva via blog.wired.com »