Saw this on the World of Warcraft tech support forum:
We addressed an issue that was preventing players on a select number of realms from logging into the game correctly. However, if you are still experiencing problems logging into your realm, we want to provide you with a quick and easy work-around.
First, open your config.wtf (located at the default, or where you chose to install the game C:\Program Files\WorldofWarcraft\WTF) with the Windows Notepad, and delete the “realmname” entry that appears. Once you have done so, please log into the game again, this time choosing a realm that is separate from the one that was giving you trouble. At this point you should be successfully connected and will be free to select your realm which previously appeared inaccessible. While it may not fix your problem the first time, please keep trying, choosing different realms in place of your old one each time.
I don’t even know what to say about a fix like that.