Bornakk broke the news earlier today that all realms will be going down, saying that Blizzard needs to perform urgent rolling restarts today, Thursday. He notes that all realms will be down for approximately 15 minutes and that the procedure, slated to start at 10:30am PST, will last approximately one hour.
However, reports indicate that restarts have taken on a different form than expected, as many realms went down several times for a short period several hours earlier. These rolling restarts, lasting no more than a few minutes per realm, occurred much earlier than Bornakk's announcement. Blizzard did not indicate what problems they will be fixing with these emergency restarts. We'll keep you posted as we learn more.