I never really expected online co-op. I remember reading before that it would be very very hard to pull off as the games need to be synced up exactly. Now, it could have been thrown in obviousally because there are other FPS's out there that support co-op online. But they wanted a flawless campaign mode rather than one that has flaws but allows you to play online.
I would think a system link co-op mode would work alot better, but theres still lots of things that could go wrong making the games out of sync, and the experience would differ based on the equipment people are using to route their connection and Bungie has been really strict on trying to give each person the best experience possible. However...
(WHAT I'M ABOUT TO POST KIND OF HAS TO DO WITH THE LEAKED VERSION, but its not much of a spoiler)..........
I remember seeing a screenshot of the leaked version and they were in "Sytem Link Mode" and one of the game options was Co-Op. Now, maybe this was a beta version or something other than the leaked version I was looking at, or maybe it was a doctored screen shot, or maybe it means something differnt all together. So take that for what its worth.