All realms are undergoing a series of rolling restarts, with each realm going down for approximately 15 minutes. The entire process should conceivably last no more than an hour. I suppose we all saw this coming considering the stability issues realms have been experiencing since Patch 3.0.8 dropped yesterday. Most realms continually crashed because of a bug in Wintergrasp but have since stabilized at the cost of disabling the battle for the zone.
Hopefully these restarts fix most of the issues that came with the latest patch -- I kind of want to actually win Wintergrasp without taking down the entire server. A series of restarts fixed the Death Knight bug yesterday, so we hope to see Azeroth return to normal after this.