Avatar: The Game sold 2.7 million copies.

[richardhutnik]

According to this Gamespot article, that is how many sales it did:

http://www.gamespot.com/news/6263057.html

Now, not sure if that it through to the customer or just on the shelves.  This site has the total sales at around half.  If this is true, it is interesting to see what other games that people rave about here don't match it in sales.