From the official WoW site (http://www.worldofwarcraft.com/):

In keeping with Blizzard's aggressive stance against cheating in World of Warcraft, we have permanently closed more than 18,000 accounts over the past three months for participating in activities that violate the game's Terms of Use.

Read the full news post and discuss this here (http://wow.forums.thedruidsgrove.org/showthread.php?p=127476).