Following mysterious updates to the original game and various teases, Valve Software has now officially announced the existence of Portal 2 through Game Informer.
Few other details on the puzzle-shooter sequel were revealed, though Game Informer notes that it is "a stand-alone, full-price retail release" with "new gameplay mechanics, storyline, and some surprising new twists."
Update: Portal 2 will hit "this coming holiday season," according to maker Valve.
Update 2: The Aperture Science BBS has been updated with the phrases "Cooperative Trial Completed" and "You will be contacted when the live first phase of the Cooperative Testing Initiative is ready to accept applicants," implying Portal 2 may sport co-op.
Update 3: PC and Xbox 360 pre-orders for Portal 2 are now open at GameStop, with the retailer specifically noting the inclusion of "single and multiplayer co-op modes. "
