Valve's sent word that the first bout of substantial*DLC*for Portal 2 will hit Xbox Live, Steam, and PSN on October 4. Entitled 'Peer Review,' it's free across all platforms. Yes, even XBL.
From the announcement:
In "Peer Review", you and a friend will continue the story of loyal bots P-Body and Atlas as you puzzle your way through a mysterious new co-op test track and once again match wits with GLaDOS[...]
Continue reading: Valve: Free Portal 2 DLC 'Peer Review' Dropping October 4...