I've experienced this first hand on a couple of my servers, and several people have been reporting it to us both via the tip line and by pointing us in the direction of the official forums.
There has been no word from Blizzard yet as to exactly what the cause is, but that'll likely change. All we know is that they're looking at it.
Just another day in Azeroth... or not.
Updated 12:55 a.m. EDT: We've received some odd reports of the letters "FU" appearing outside of Violet Hold in Dalaran on some severs, and of pictures like this. Quite odd. There's a possible story developing here, or it could just be very wacky server behavior. Blizzard is apparently aware of the picture linked (and many like it), and is working towards a solution.