Valve has announced (via Destructoid) that the
next free update for Portal 2 will include the Perpetual Testing
Initiative, which is fancy Aperture-talk for what we in the
shamefully Portal-poor world call “a level editor.” The PTI
update, due for release as a free Steam download come May 8, will add a finalized version of the
simplified map editor that saw beta-testing last year.
Above: That very editor in all its beta-version glory
Maps created within the
Perpetual Testing Initiative will be available for sharing on the
Steam Workshop (in case you'd gotten this far on hope and fumes, it
should be clarified for definite that the PTI content's bound only
for computer players). Valve's also said that some maps will get
bundled into the game itself, though presumably that initial idea you
had for a map shaped exactly like Mario's head might need a little
fine-tuning if it'll make the cut.