Happily, the internets were wrong. Via Joystiq
comes the happy news from Bungie that not only will we be able to play the Halo 3
campaign in online co-op mode, but up to four players at the same time
can enjoy the experience. "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."
For more on the game, 1up is running the only preview out there of an entire single-player campaign level
called 'Tsavo Highway'.