Drysc reports that a number of realms are under investigation for performance issues and will be undergoing a scheduled maintenance at 5:00 AM PDT on Wednesday, March 12 with a foreseeable downtime of 1 hour (update: Zorayn notes that downtime will be extended to 2 hours as opposed to one). As Drysc edits the thread, realms are added (or removed) regularly. From a quick perusal of the realms, it seems as though it is the Nightfall Battlegroup that is affected. Blizzard currently lists the affected realms as follows:
Altar of Storms
The Venture Co
Drysc further notes that other Battlegroups are under close observation and will be taken down for maintenance should any issues arise that might require it.