Buried in Bungie’s weekly update is a terse confirmation of an unprecedented feature for the series: 
“Totally Unrelated Question 4: Can I haz Campaign Matchmaking?” writes Bungie. “Totally Related Answer: Yes.”
No other details.
What this means: Halo 3 supported multiplayer campaign but not matchmaking for it. So you could hop into a game with a friend over Xbox Live but couldn’t search for them. Halo: Reach will instead feature its own campaign playlist, allowing you to find your pals (or team up with anyone at random.) This will be especially handy for later levels or harder modes where you need a big friend to help out.
Bungie Weekly Update: 8/20/10 [With credit to ThatVideoGameBlog for noticing what many missed in skimming.]
Dire Wolf
Monday, August 23, 2010 at 8:55 AMAwesome. I am so exited for this game.
The Cracks
Monday, August 23, 2010 at 10:07 AMI’m guessing this was added primarily because they added matchmaking to Firefight, and firefight largely uses campaign code to function.
Stinky
Monday, August 23, 2010 at 10:54 AMBottomless pit of gameplay.
706-2
Monday, August 23, 2010 at 11:58 AMI never even considered jumping online to play some campaign with random people. I always want to play more objective style multiplayer games, this may be that chance. Hopefully players would work more as a team.
James Mac
Monday, August 23, 2010 at 12:01 PM“This shit just got real” /END Martin Lawrence}
Reece Offer
Monday, August 23, 2010 at 5:46 PMI can’t believe they didn’t think of this sooner