The announcement on the login screen tells us that there'll be rolling restarts for all realms between 5:00 a.m. PDT and 6:00 a.m. PDT. Each realm should be down for no less than 15 minutes.
These restarts have become a common thing in the past few days. From previous information given to everyone from Bllizzard, we know that restarts correct server lag and other connectivity issues. And there have been quite a few of these issues since patch 3.0.2. A restart fixing these things is nothing new. But you have to ask yourself now, are these updates secretly pushing new things onto the server?
It seems like for the past few days after every "rolling restart" a new phase or two of the scourge invasion has begun.
So I'm going to make an educated guess based on what may very well be completely unrelated variables – tomorrow or Monday we'll see phase five of the invasion, complete with something new we haven't yet seen.
Prove me wrong, Blizzard... prove me wrong.