http://www.shacknews.com/onearticle.x/51482
Linking this article for a reason. In it Valve says that The Orange Box sold over 1 million on 360 and another few hundred thousand on PS3 (reporting shipped of course) but that the PC sales of the game were signifficantly higher than both combined by the double digit percentages. This implies that The Orange Box has sold well over 2 million on PC, and much of that was on Steam. So yes, as Shio said, CoD4 sales on PC could easily be well over 1 million now given how it's charted in sales tracking.
Lombardi goes on to say the PC market is healthy, but not in American retail sales. He says in other countries like Germany PC games outsell console games in retail and digital distribution is thriving.
