A post to the Infinity Ward Forum by Robert Bowling explains that in spite of the uproar from PC users expressing discontent over plans for Call of Duty: Modern Warfare 2, the sequel to Call of Duty 4: Modern Warfare has outsold the original in its first week:

Yes, PC is the smallest percentage in terms of how much sold on each platform but that hardly means anything other than the PC is just the smallest market.

The PC version of Modern Warfare 2 has actually outsold the PC version of Call of Duty 4: Modern Warfare in it's first week. Making it the most successful PC version.

Essentially, all this percentage proves is that the console versions sold great, not that the PC version sold poorly, because that's actually not the case. The PC version has done tremendous in it's first week, better than our previous game.