Most realms are having regular scheduled maintenance starting at 3:00am PDT, and they'll be coming back online around 11:00am PDT. However, there is a huge chunk of servers that have already gone down as of 12:01am PDT and will be down until 11:59am PDT. My assumption is that the extended downtime will be equipping these servers with the new instance hardware we've been hearing about. You know, the stuff that will soon allow Cross-realm LFG? Yeah, that.
Click through the break below for the full list of realms that will be down for extended maintenance, but chances are if you play on those realms, you already know about it.