Blizzard says that they're performing additional maintenance to resolve some issue. However, unlike most every other time they've taken down realms, this time there is no ETA on when they'll be back up.
We can only hope that this will be a quick fix problem. I had a dream that Kaz'rogal will be dropping Kaz'rogal's Hardened Heart for me finally, and I'm going to be mighty disappointed if I can't raid tonight and get some of that phat lewtz.
Take a look after the break for the long list of realms that are offline. And remember, stay tuned to WoW Insider, we'll let you know when these realms are back up!
Updated 8:01 EST: The realms down are now only as follows: Alextrasza, Balnazzar, Blackhand, Cho'hall, Destromath, Dethecus, Garona, Gorgonnash, Gul'dan, Hellscream, Illidan, Kael'thas, Kirin Tor, Ravencrest, Smolderthorn, Spinebreaker, Stormreaver, Whisperwind. The rest of the realms are back online!