On the WoW community site (http://www.worldofwarcraft.com/), Blizzard posted that they're doing a stress test beta soon. 100,000 people will be able to play for 7 days to help test the WoW servers under high loads.

See more information on this, and discuss it here (http://wow.forums.thedruidsgrove.org/showthread.php?p=114061).