So much for "mid-September," eh? Valve has announced that the first DLC for Portal 2, which is called "Peer Review," will be available for free download on PC, PS3, and Xbox 360 on October 4.
"Peer Review" continues the cooperative exploits of Atlas and P-Body, the two beloved robot protagonists in Portal 2's co-op campaign. GLaDOS will also be featured as the two tackle a new co-op test track. However, the DLC will also include a Challenge mode for both co-op and single-player with full leaderboard support.
To help ease the pain of waiting for free stuff, Valve's offering, uh, free stuff! You can download the third and final group of tracks from Portal 2's soundtrack, Music to Test By, right now by heading on over to Valve's site here.