Starting at least half an hour ago, and continuing through to now, 36 US realms, comprising the battlegroups Rampage and Shadowburn, are down. Blizzard notes that they are "currently unavailable and are being worked on," and that they don't have an ETA right now, but will give us an update around 8:50 AM PDT/11:50 AM EDT. So we should hear more about this in 15 minutes or so, at which point I'll update the post with any new information.
The affected realms are: Agamaggan, Alexstrasza, Alleria, Azshara, Baelgun, Balnazzar, Blackhand, Cho'gall, Dark Iron, Destromath, Dethecus, Detheroc, Emerald Dream, Garona, Gorgonnash, Greymane, Gul'dan, Hellscream, Illidan, Kael'thas, Kalecgos, Kirin Tor, Lightninghoof, Maelstrom, Malfurion, Moonrunner, Nazjatar, Ravencrest, Sargeras, Spinebreaker, Staghelm, Stormreaver, Twisting Nether, Ursin, Whisperwind, and Wildhammer.
Update (12 PM): Blizzard provided an update to let us know that they will provide another update at 10 AM PDT, which is in an hour. They're working on it and appreciate our patience.
Update (1 PM): Now they think they've figured out what's wrong, and they're saying the realms should be up by 10:30 PDT/1:30 EDT.