If you're playing Diablo 3, be VERY WARNED. There is a rise in account hacking for the game already. According to some sources, this can even happen if you're using an authenticator to ward off hackers OR if all you ever played was the Single Player campaign. Some people claim that they were online at the time they were hacked and there was never any evidence that they were hacked at any time, which indicates some kind of exploit people are using to hack the accounts.
A theory of how that's happening is that if someone happens to figure out your session identifier, they can somehow screw with their client so it sends your session identifier instead of their own, which bypasses the need for your password or username.
If you're wondering how bad it is, look
here and
here. Even REPORTERS have had their accounts hacked. And as that second link says, if you've got someone you know you've never played with in the list of people you've played with (some of the most common usernames found being "leiyong", "Nevin", "SBJunkie", "lucklezz", or "McLeast"), then you could have been hacked.
Basically, all of Blizzard's responses are the stereotypical junk that doesn't help, like "Change your password and get an authenticator" or "You may have logged into the wrong region."
Here is the second thread about it on the battle.net forums (the first one hit the post limit).
Source