Source (no pun intended): http://www.tomshardware.co.uk/Deep-Steam-Valve-Software-Shane-Acker-Brown-Bag-Films,news-38832.html Gotta say, this is quite a step for Valve. Perhaps the next evolution of Machinima?
Cheap way to make a film I suppose, I don't think Disney or Dreamworks will be too worried. Turning the sets into playable levels for a game makes some more money.