That's another reason why I think that these "brute force" hacks don't happen very much. I mean, if you are just guessing someone's pw, even if it is all numbers, that should take an incredibly long time to pull off. Even using a sophisticated program. I log in often enough that I would probably notice if someone has attempted to break their way into my account, and I would act accordingly. I think most of these hackings are due to keylogs, viruses, ect.
If these brute force hacks are common, it would be fairly simple to stop them. After two or three unsuccessful attempts to log in, the account should be shut down, and only activated when Blizzard receives an e-mail from the e-mail address that was registered.