We knew it was going to be a lot, but I think the game outsold even our expectations: Blizzard has announced that the game's second expansion, Wrath of the Lich King, sold 2.8 million copies worldwide in the first 24 hours, and undoubtedly many more in the weekend after that (we're expecting an announcement later this month around five million in the first 30 days). That makes the game the fastest selling PC game of all time, and by far the fastest-selling expansion of all time (remember that this isn't even a complete game that's flying off the shelves). The previous record, of course, was set by the Burning Crusade, which sold 2.4 million copies during launch.
Pretty huge, but when you consider that the game has 11 million subscribers around the world, those numbers are just about right. Looks like Morhaime is on to something -- as long as they have players ready to buy the game in numbers like this, Blizzard will undoubtedly release expansions as long as they can.