According to Bowling, PS3 owners can expect a "party invite update by Friday," which should hopefully resolve this issue. Until then, players will have to make due with the single-player campaign. It's not like this "multiplayer" thing is important, right?
Update: Tipster Kenneth B. pointed us to a tweet from Activision support that says the MW2 outage is "totally unrelated to the patch." While this means a fix can be done before Friday, he admits "we don't have a hard ETA on the servers coming back up." We'll be keeping track of this problem, but don't forget to send us your tips.
Update 2: Tipster Brent point us to yet another tweet, this time from Robert Bowling who confirms the multiplayer downtime is caused by server overload. According to Bowling, "Xbox Live servers are maintained and ran by Microsoft. PSN servers are ran and maintained by 3rd party companies hired to do so." Seems this mysterious "3rd party" infrastructure simply cannot match the Microsoft-funded backbone of Xbox Live.
Update 3: It appears everything is finally fully operational. For more details, check out this updated story.