http://us.battle.net/d3/en/forum/topic/5149619846?page=29#571
I'm inclined to believe it, most people are very lax with their home network security. Their account details and passwords were probably stolen some time ago, now with the release of Diablo III, the thieves are making use of those details. It could have been stolen in a multitude of ways. A flat out weak password, stolen from another compromised site with the same password (most people use the same password for everything), ye olde' traditional keylogger from installing garbage etc. etc.
If your Battle.net account has been compromised, there's a good chance whoever got into your account could get into your Facebook, Gmail, online banking etc.
I always suggest a complete format, re-install and change of passwords across the board to people who get any of their accounts hacked.