The good news: Modern Warfare 2, over four million copies sold. The bad news: over four million copies illegally downloaded. Like music and film, games are just data that can be sent over the tubes. Film and games are respectively harder thanks to their larger size, but growing bandwidth will make this point moot. Piracy is getting easier and easier, and the PC gaming section in the retail stores looks more and more pathetic. While not cause and effect, these are not unrelated either.
Inevitable result: players must log into an internet account to validate their purchase and play a game, much like you would an MMO. Of course, a large percentage of PC game sales these days seem to be un-pirateable MMOs anyway.