Infinity Ward's Community Relations Manager Robert Bowling runs his own blog. This week he talked about some of Infinity's Call of Duty 4 projects and he also explains why game developers aren't making PC games anymore.
He didn't post any numbers but says the piracy rate of games like COD4 is astounding:
On another PC related note, we pulled some disturbing numbers this past week about the amount of PC players currently playing Multiplayer (which was fantastic). What wasn't fantastic was the percentage of those numbers who were playing on stolen copies of the game on stolen / cracked CD keys of pirated copies (and that was only people playing online).
Not sure if I can share the exact numbers or percentage of PC players with you, but I'll check and see; if I can I'll update with them. As the amount of people who pirate PC games is astounding. It blows me away at the amount of people willing to steal games (or anything) simply because it's not physical or it's on the safety of the internet to do.