Via Ars Technica by way of Game Informer.
No real details have been revealed, although the game will be a "full-fledged, standalone sequel," as opposed to the original, which came packaged with The Orange Box.
I wonder when the complaining starts.
Edit: Coming during the "holidays" according to Valve.
Double edit: And I guess this means I better get Portal, then. Any good things to say about the XBLA version?