Bornakk posted on the forums about rolling restarts for all realms to affect an issue that was preventing the proper display of realm names in the Battlegrounds. Instead of realm names, players from other servers would have their names displayed with cryptic tags such as US1B-BG, which -- aside from being unintelligible -- prevented identification of premade groups. The issue had reportedly been present in the PTR builds but seems not to have been addressed completely when Patch 2.4 went live.
Although the rolling restarts were estimated to affect each realm for approximately 15 minutes and take a total of around 2 hours for the entire process, some realms -- roughly half of all US servers -- continue to experience further issues, necessitating more downtime. As of this writing, Bornakk reports that Blizzard estimates that all realms will be available for play at 4:30am PDT. Interestingly, it seems that the issue is not Battlegroup-related as some Battlegroups have both up and down servers.