No word on the SDK, but Portal 2 DLC will be coming soon with a price tag of $0.00!
"Valve, creators of best-selling game franchises (such as Half-Life, Left 4 Dead, Team Fortress, and Counter-Strike) and leading technologies (such as Steam and Source), today announced the first game add-on for the year’s highest rated multiplatform game, Portal 2, is in development and targeted for this summer.
Portal 2 DLC #1 will offer new test chambers for players, leaderboards, challenge mode for single and multiplayer modes, and more.
Targeted for release this summer, Portal 2 DLC #1 will be offered free of charge for the PC, Mac, Xbox 360, and PS3."
Source: RockPaperShotgun and Interlopers.net
EDIT – Oh snap, and Challenges are making a comeback! Hooray!
The new DLC pack, entitled DLC #1, will be released this summer for the incredible price of free! The pack will include new test chambers, leaderboards, and challenge modes for both single and multiplayer. The DLC will be released for free on all platforms, so you can continue playing with your PS3 friends (if they’re still using it after Sony’s security leak fiasco).
Source: PC Gamer