Bungie has responded to an out of control rumour which stemmed from a leak from an upcoming issue of US video game magazine EGM. You may have seen headlines such as "Halo 3 Co-Op Heads Offline" but that's not entirely true.
It's not that the co-op heads offline, but that online play is still being implemented and may not make the released game.
"All we confirmed to EGM ... regarding Cooperative Play in Halo 3's Campaign is that System Link co-op will be included in Halo 3," said Bugie's Frank O'Connor. "Just like we told EGM, we're aware that people want co-op online but the challenge of getting our complicated A.I. systems, huge game world, and gargantuan encounters neatly packaged and transferring across the Interwebz is a mythic undertaking."