Word of a sequel surfaced around June of 2008 when news broke that Valve was hiring an actor to voice Cave Johnson, who was speculated to be the title's main antagonist. Valve fueled anticipation of a new game this week, after a couple of updates to Portal -- one of which introduced new ending.
Putting rumors to rest, Game Informer has now revealed that its April 2010 cover story will be Portal 2. Very limited details have been given, but the game will be a "full-fledged, standalone sequel" with "new gameplay mechanics, storyline, and some surprising new twists."
The magazine will ship with a 12-page feature on Portal 2, and it should hit newsstands soon enough. Also, Valve says the title will launch this holiday season. In the meantime, you'll have to make do with the provided cover shots, which you can view in full here.