Valve has confirmed that the first batch of downloadable content for Portal 2 will be free of charge on all three of the platforms - yes, even the Xbox 360.
The pack will introduce some new test chambers, and a challenge mode which supports both the single-player and co-operative game modes. There will also be online leaderboards added for this new mode.
Valve didn't give a more firm release date for the content, so it's still stuck with a "summer" release date. Also, feel free to check out our review of Portal 2.