"According to a tweet by Infinity Ward's Robert Bowling, a "trophy update" has been released. However, while said update fixes one problem, it apparently creates another. "It killed all multiplayer activity on MW2," tipster True_Darknight tells Joystiq."

PS3 Modern Warfare 2 experiencing multiplayer issues; patch expected Friday [update]

Edit: well it's caused by a different, more lulzy reason:

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.