Halo: Reach will support co-operative campaign matchmaking, you just haven't seen the feature yet because Bungie doesn't want people getting the story spoiled on the first mission they play.
But in a couple of weeks, there should be enough of a gap since release that most folks will have seen the campaign plot points on their own already, Bungie writes in its weekly update. Expect the playlists to come in early October on the first matchmaking update for reach. Other game types and Forge maps will be available as well.
That said, the potential for spoilers still exists if you hook up with a more advanced team. Missions may be vetoed like any map in standard multiplayer, but if the majority overrules your desire to play a part of the campaign well ahead of your singleplayer progress, you'll have no choice to bail out before it begins.
In summary, "If you're worried about spoilers make sure you finish campaign before jumping into this playlist!" Bungie writes.
No specific date was given, just "early October."
Bungie Weekly Update 9.17.10 [Bungie]