Titled "You Monster" featuring GLaDOS. There will be new levels and challenges. The expansion will cost 4.99 or 400 MSP and is releasing on December 7th. View the trailer here http://www.youtube.com/watch?v=n7bkewdgjKA
I just hope this will cause Giant Bomb to fix the achievements for the base game, as they have been screwed up for quite some time.