7 million copies in the first four days on sale (Batman took 18 months to sell 2.5 million copies by comparison).
What does this prove? Well firstly, that James Cameron has done it yet again. He's made lots of money, yadda yadda yadda. But it also shows that Blu-Ray adoption is pretty sharply on the rise. In fact, Blu-Ray adoption accounted for 40% of Avatar's home-video sales in the four-day window.
Presumably Avatar will get another boost when the 3D Blu-Ray is released next year. And Playstation 3 owners will have one of the few Blu-Ray players on the market capable of viewing the movie the way Cameron intended.