We're not out of the woods with this patch yet. I'm seeing this message on a North American realm, and lots of other people are seeing the same thing. At least one of our bloggers is still in the game, though, and Blizzard is saying the Authentication servers are still up and running, so this should be fixed soon.
We'll keep you updated, stay tuned.
Thanks to everyone who sent this in!
Update: A login server died. If you try signing in with a Bnet account, you should be able to get in. Otherwise, everybody
Update 2: Everything seems back up. That was relatively painless. Thanks for playing, everybody!