If you have a Battle.net account, chances are that hackers may have their hands on some superfluous information relating to your account. According to Blizzard:
“…our security team found an unauthorized and illegal access into our internal network here at Blizzard. We quickly took steps to close off this access and began working with law enforcement and security experts to investigate what happened.”
They claim that no information that could pose an immediate threat to your money or identity was taken.
“…we’ve found no evidence that financial information such as credit cards, billing addresses, or real names were compromised.”
If this is true, then your pockets are safe. Some information was indeed stolen, but so far all that they’re sure has been taken are: some passwords, which have been scrambled by a “Secure Remote Password Protocol”; some email addresses; your secret questions and answers; some ‘Mobile Authenticator Data’ and ‘Dial-In Authenticator Data’; and ‘Phone-Lock Data’ (a security system used in Taiwan). They still advise you to change your password, in case the passwords for your Battle.net account are decoded. As for your secret questions/answers: there is nothing you can do to change this. They will prompt everyone with a chance to change their question and answers soon.
All in all, they’ve done a good job of alerting everyone of the situation immediately, in contrast to some previous incidents where it took quite some time for the company to admit that something had gone wrong.