Apparently if you want to authenticate your game tonight, you'll be out of luck. At least for the time being.
Authentication? Logging in? Who needs that?
There has been some conflicting information out there. According to Bornakk, all the issues have been resolved. However, according to Billtesdes the authentication issues are being investigated.
Judging by the number of you sending in tips to WoW Insider, I'd say things are definitely broken at the moment.
We'll update this post when more information is available.
Update: This issue is apparently fixed.