Apart from the announcement over at steam, IGN also received some more information regarding the game:
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."1
On top of that, GameInformer will be having a hub page for the game showing supplementary information through out the month of march, see here.