Valve officially announced today that http://www.trueachievements.com/game.aspx?gameid=3532
will be receiving additional downloadable content in the near future.
While not much in the way of details has been made available at the present time, Valve has confirmed that Portal 2
DLC #1 "will offer new test chambers for players, leaderboards, challenge mode for single and multiplayer modes, and more".
They also confirmed that the DLC has a Summer launch window and that it will be absolutely free of charge.