In an interview on tonight's X-Play, Kimberly Swift, the lead designer behind portal, confirmed that a sequel is in development.
"This is pretty much a given as Doug Lombardi, Valve's Director of Marketing, had previously stated that "more Portal is definitely on the way." However, until now we weren't sure if this would be in the form of a direct sequel or possibly downloadable content. Well, now we know. Check out the video for the confirmation."