Update:  The DLC is apparently available now on Steam.  Check it out.

Tomorrow, the new content pack for Portal 2—dubbed Peer Review—will launch on all platforms.  It features new co-op maps so Peabody and Atlus can continue their testing adventures for science.  Also included are challenge modes for the single player and co-op maps.  The Peer Review designation is most likely referring to the leaderboards, which is also being added to the game.  So now your failures can be openly mocked by your friends and GLaDOS at the same time.  Awesome!

[via PC Gamer]