As of 10:50 AM PDT, 1:50 PM EDT, Blizzard has brought down all the realms in the Bloodlust (US) battlegroup for brief maintenance "to address issues relating to high latency and disconnections." The affected realms are expected to be done after about an hour, which means they should come back up around 11:50 AM PDT/2:50 PM EDT, which is in twenty minutes.
The affected realms are listed as: Aman'Thul, Barthilas, Blackrock, Caelestrasz, Dath'Remar, Dreadmaul, Frostmourne, Frostwolf, Khaz'goroth, Kil'Jaeden, Kilrogg, Nagrand, Ner'zhul, Proudmoore, Sen'Jin, Silver Hand, Thaurissan, Tichondrius, and Vek'nilash.
Note that as far as I can tell the Dreadmaul realm does not actually exist. Edit: Never mind, it's the new Oceanic realm. It's just not listed on the Battlegroups page for some reason. I kind of wish they'd do some maintenance on my own realm (Shadow Council), since I've been having relatively high latency myself recently (150-200ms as opposed to my usual 60-100ms).
Update (3 PM): Not entirely unexpectedly, the realms are not fixed yet. The new ETA from Bizzard is 1 PM PDT/4 PM EDT.
Update (4:30 PM): The realms are back up, but still experiencing issues. Blizz promises to provide more updates as they are available.
Update (8:15 PM): Realms down again. They were supposed to be back up at 5 PDT/8 EDT, but apparently that is not the case.