The login server has announced that Blizzard is aware of some authentication server problems going around tonight. It didn't seem very widespread when the notice first went up, but we've been gradually getting more reports of it as the night has gone on.
Oddly, I was able to log into WoW when the notice was first posted, but I can't anymore. I wasn't doing anything important, so I decided to check it out and sure thing, it was time for a ride on the failboat. I'm also stuck on a loading screen while trying to test out the Iron Council on the PTR, so I'm currently riding on a Cruise Failboat. How decadent!
If we get any other updates before the authentication server starts working again, we'll be sure to keep you all posted.
Edit: Welp, it works again. Go forth and play!
Edit: Welp, it (mostly) works again. Go forth and play (some of you)!