Sequel set for this year. Xbox 360 and PC only.
Valve has been teasing the sequel for the past week, updating the first Portal on its Steam client with new Morse code sound files, and even a new ending sequence.
Update: Gamestop's listing for the title brings new information. According to the product page, the game "introduces a cast of dynamic new characters, a host of fresh puzzle elements, and a much larger set of devious test chambers," it reads. "Players will explore never-before-seen areas of the Aperture Science Labs and be reunited with GLaDOS, the occasionally murderous computer companion who guided them through the original game."
GameStop currently lists the title for Xbox 360 and PC. No PS3 version is coming. The title is due out on October 26, 2010, though that release date may not be final. It's also a full retail release. The Xbox 360 version is $59.99, while PC is $49.99.