Testing begins anew next week, guys.
Valve just sent word that Portal 2's first set of downloadable content, dubbed Peer Review, will be released on every platform on October 4.
And if you've forgotten, this content won't cost you a dime.
Peer Review contains both single and co-op content, though the additional "story" bits appear to be relegated to the co-op expansion, which continues to follow P-Body, Atlas, and GLaDOS.
If you're playing solo, Valve's included a Challenge Mode with leaderboard functionality. Said Challenge Mode is also available for co-op play, though. Co-op! Co-cop! Co-op!