We started getting a few tips about realms going down about half an hour ago, and Blizzard's now confirmed that 3 dozen realms are down to address "some instancing issues." Interestingly, the brand-new realm Borean Tundra is among them, and I recognized a few others that are relatively new, as well as the usual forum whipping-boy Misha.
2 hours is the estimated downtime, so if you're on one of the affected realms, I'd start checking back around 2:45-3:00 pm EST. We'll keep you up to date if maintenance is extended. Read on for the list:
UPDATE 2:03 pm EST: Anub'arak is now listed as the only realm affected.
UPDATE 2:32 pm EST: Looks like all the affected realms are back up early.