Originally Posted by NarcissistZero
They support the 360 because they like making their games available to as many people as possible. That is why Source has low spec requirements, and why Steam makes it easy to join an online game. Valve is all about including people, and Natal would alienate like 90% of their audience.
That is assuming that it would require Natal instead of being an extra feature. I remember seeing a demo of Natal running HL2.