Blizzard has announced that they are conducting urgent maintenance on several realms.
Though the official Warcraft twitter account announced it would only go on until noon PST (the tweet was about account maintenance), the in-game and forum messages state that they do not know how long the maintenance will take. They expect to have more information by 12:30 p.m. PST / 3:30 p.m. EST.
The list of affected realms is after the break (which includes Zangarmarsh, the It came from the Blog server).
We will keep you informed as the status changes.
3:25 p.m. PST: All realms are (supposedly) online and playable.
Update 3 p.m. PST: More info at 3:30 PST.
Update: More info at 1:30 p.m. PST.