Today is a Wednesday, and that means that there has to be some sort of Battle.Net authentication/login/game-version issue today. And guess what? There is!
There is a long thread in the support forums about it, and the only blue response so far is that Blizzard is aware of the problem and they're working on it. Many, if not all, Battle.Net users are seeing an error stating the client cannot validate the game version (even after they've patched to 3.1.1).
There is no known fix yet. We'll update this post when there is more information to report.
Update: According to Neth everything is working now.