Looks like Geoff Keighley might be having a "Downfall" moment as another of his coveted GTTV 'exclusives' leaks online. The Portal 2 DLC has been revealed as the long rumored puzzle creator. "Perpetual Testing Initiative" is set for release on PC & Mac on May 8th.
The DLC will allow players to create new puzzles, share them and vote on other's puzzles. Unfortunately this DLC won't be coming to consoles likely because of Microsoft requiring all DLC to cost something.