I just tried to play Diablo 3, but could not log on due to server maintenance. Blizzard requires you to be always online to play even single player campaigns. I’ve seen a lot of people rage about this, but Blizzard actually has very good reasons to do this. And they even kept you, as a player, in mind!
First of all, what Blizzard is doing is not really DRM.
Digital Rights Management is making sure you have authorization (from the providing party) to do something off-line.