Fourteen US realms were taken offline at 11:07 AM PDT in order to "troubleshoot technical issues." While Blizzard hopes to have the realms back online as soon as possible, there's no current ETA. Impacted realms include Aerie Peak, Altar of Storms, Anvilmar, Blackwing Lair, Deathwing, Demon Soul, Doomhammer, Gnomeregan, Icecrown, Jaedenar, Kel'Thuzad, Lethon, Tanaris, and Uldaman. Comments on the realm forums seem to mention recent lag and stability issues, but CM Tseric mentions no specific reason for the realms being pulled offline.
Update: These realms should be back online as of 11:51 AM PDT.