Taken from our "Extended Maintenance 4.0.1 Release - 10/12" sticky thread:
http://forums.worldofwarcraft.com/thread.html?topicId=27189089214&sid=1

Q u o t e:
Update (5:00 p.m. PDT):

We’re in the process of finalizing our tests on all realms and are extending the anticipated time for bringing them back online to 6:00 p.m. PDT.

We are working on getting them up as soon as possible. Please keep an eye on the sticky thread linked above for additional updates and please refrain from posting the same topics. This only congests the forums.