Over a half a million users have registered for Warhammer Online to this date.  Matt Martin from gameindustry.biz has the figures in hand:

The figures come after only one week on sale, with EA claiming the title is the fastest-selling MMO of all time.

"In just one week we have a half a million people playing WAR online, and the ranks of Order and Destruction are growing at a record-breaking pace for a new MMORPG," offered Mark Jacobs, co-founder of Mythic Entertainment.

"We spent years working to provide players with the most stable, epic, and polished online world we could, but it is the players that have truly brought the Age of Reckoning to life."

A discussion topic has been posted in our forums , marking this historic occasion.  Waaagh!