Currently, it appears that Halo 3 will ship without online co-op, but supposedly Bungie hasn't ruled out bringing the feature to players through a future update (remember when folks wished for the same for Halo 2?). This doesn't mean that 4-player co-op is any less likely; but, if it does exist, you'll be forced to suffer through it using antiquated split-screen methods (unless you've got a LAN setup). Disappointed? We are; many of us naively accepted Gears' online co-op as surrogate confirmation that Halo's campaign would finally become an Xbox Live experience.
We'll leave you with a few hopeful words from Frankie: "If we can make it happen in a way that works well, we will."
Update: 1UP editor Jeremy Parish weighs in on the controversy; Update 2: Bungie's Frankie says, "like other elements of Halo 3, online co-op is a feature we're wrestling with ... as we approach the tail end of development, we will have a clearer picture of whether or not that online feature will be included."