Activision & Valve team-up!
 

Activision and Valve have teamed up to provide Call of Duty 2, Call of Duty, and the United Offensive expansion pack via Valve's online digitial distribution engine, the oh-so-popular Steam

As more and more companies recognize the trend in online distribution, Activision has chosen the leader of the movement for their distribution rights. Steam was one of the first programs to offer online distribution for purchases, and it is definitely the leading company on that front.
Awesome deal!! If you purchase Call of DUty 2 via Steam before October 27 you get a 10% discount! So rather than paying $39.95 for the game, you only pay $35.95!

so i heard, old news nontheless, anybody who has steam would already know. :p