It looks like Diablo III, like all Blizzard games, have been heavily targeted by hackers.
I believe the old pitfalls are to blame. Keyloggers, weak passwords and lack of authenticators.
Not saying there isn't a security hole. The ammount of server restarts and maintenance is worrying, but you this doesn't means the tried-and-true methods of account hijacks are outdated.
I hate the idea of using an authenticator but I bite the bullet and I'm downloading one today.
However, I admit the people who really hated the whole always-online thing have been given plenty of ammo to shoot lately.