Edit: The 64 bit client has now been released, please see here for details:
http://eu.battle.net/wow/en/forum/topic/3312962371#1

We are working on getting the new 64 bit client released to you as soon as possible and are finishing up some last minute preparations.

At the moment, if you have downloaded the 64 bit client, please do not use it as you will be unable to log in unless you use the 32 bit version.

We will update here as soon as we have any news or updates regarding this client.