UPDATE: All fixed. According to a post on Bungie.net, "The online team tells me the source of the problem was rooted out, physically destroyed, and tossed into a dumpster. Again, apologies for the early morning hiccups."
ORIGINAL STORY: Halo: Reach is suffering a mysterious matchmaking outage, developer Bungie has confirmed.
A post on the developer's official site explains that it doesn't currently know the cause of the problem but is working hard on a fix.
"Our Test team is furiously scrambling to determine the source of the outage and to make the necessary repairs. As of right now, they are not able to supply an ETA, but rest assured that Top Men are on the scene.
"Apologies for the imposition. Thank you for your patience. We'll notify you when everything is back online."