And who will be on the cover of Portal 2 will be the robots, who will participate in the co-op mode (the story single player continue with Chell).

The game is scheduled to be released in April 21st and will go to PC/Mac (should be on the same disk), Ps3 and Xbox 360.
There was also a secret Christmas video, which unfortunately I didn't see at the time. Check it out:

[youtube=http://www.youtube.com/watch?v=bMq8KNauXdQ&w=640&h=385]
Source [Kotaku BR]