Rolling restarts are currently in progress on many realms, apparently to address a talent issue that cropped up with patch 3.1.3. These restarts shouldn't take more than 2 hours across the board. Meaning that your server shouldn't be down for very long at all, because that two hour window is for the entire process. The battlegroups being hit with these restarts are:
Update: They're still working on getting the downed realms into working order, but don't currently have an ETA. They say an update will come soon.