On top of the already extended maintenance today, many US realms, as it turns out, will be undergoing even more extended maintenance. The ETA for completion on this is 1:00 PM PDT. The Reckoning, Vengeance, and Bloodlust battlegroups are affected. This includes my own realm -- there go my plans for leveling my druid this afternoon. Realms not on this list are currently up and available for play, so maybe I'll go hang with ICftB on Zangarmarsh.
The complete list of affected realms is: Aegwynn, Akama, Aman'Thul, Arathor, Barthilas, Blackrock, Bonechewer, Caelestrasz, Chromaggus, Dath'Remar, Dragonmaw, Drak'thul, Draka, Dreadmaul, Eitrigg, Eldre'Thalas, Firetree, Frostmane, Frostmourne, Frostwolf, Garithos, Gurubashi, Hakkar, Jubei'Thos, Khaz Modan, Khaz'goroth, Kil'Jaeden, Kilrogg, Korgath, Kul Tiras, Malorne, Mug'thol, Muradin, Nagrand, Nathrezim, Ner'zhul, Proudmoore, Rexxar, Runetotem, Scarlet Crusade, Sen'jin, Shadow Council, Shadowsong, Silver Hand, Silvermoon, Skywall, Smolderthorn, Spirestone, Terenas, Thaurissan, Thorium Brotherhood, Tichondrius, Vek'nilash, Windrunner. Again, these realms will allegedly be up at 1:00 PM PDT/4:00 PM EDT.
Update (4:15 PM): All realms appear to be online and operational.