It looks like today's maintenance is being drawn out because of "concerns that have arisen with Cross-Realm Battlegrounds" (at least according to the current login message). So instead of coming back up at 11:00 AM PDT, we're looking at a 1:00 PM PDT uptime for all US realms. However, as of the time of this writing, several US realms have already come back online, so if you're wanting to play, it doesn't hurt to check. Online by the usual 11:00 AM are: The Forgotten Coast, Anub'arak, Cenarius, Lightbringer, Uther, Cenarion Circle, and Hyjal. The rest of us are just going to have to wait!
Update: While numerous realms have come back online, officially, maintenance has been extended until 4:00 PM PDT.