In the last fifteen minutes or so we've received numerous reports that authentication servers are down for North American realms. While the outage does not appear to be affecting anyone in game, those folks logging in will be stuck at "authenticating" for the time being.
There has been no word from Blizzard about this, but if we've gotten a dozen tips about this in the past fifteen minutes, I'm sure they have too.
We'll update this post when we have more information.
Update #1: Speak of the devil, there's a blue post about this now.
Update #2: It appears the issue is now resolved.