I had been getting that message every time I'd try to log in and it would happen during "Authenticating Credentials". I literally got it to log in twice and I was getting fed up with it, seeing as how I got it on launch day and have only put in about an hour or so on it.
What I did was I went into my router's settings on my browser, (Linksys WRT110), went to Applications and Gaming > QoS, and then where it says "Internet Access Priority", there is a drop down list for the category of the application you want to have a higher priority, so I clicked "Online Games". For the "Applications" drop down menu, I clicked "Add a New Game", and typed in Diablo 3 as the name. Typed in the ports to open, (80 TCP, 1119 both, 6881 - 6999 both). On the "Priority" drop down menu, I clicked "High", though it might only need medium. Clicked save, tried to login, and it worked instantly.
Hopefully if you guys try this, you can get the same results as I did, because I hadn't been able to log in since the 16th up until now.