Of course, the possibility remains that some aspect of Diablo III's current operations is placing excessive strain on other areas of Battle.net's infrastructure, and the targeted blame may yet be accurate. We have yet to see a statement from Blizzard on the downtime, but we will update you as soon as we do.
Update: The game-side issue appears to be an issue with the login server. If you are already logged into World of Warcraft ... don't log off if you want to keep playing!
Update #2: The Battle.net downtime appears to have been resolved.