According to this message from Mike Morhaime, Blizzard’s internal network has been at least partially compromised.

Bad news indeed, and as per the advice in the message, you should update your password now, if not sooner.

The security breach affects players on Blizzard’s North American servers (North America, Latin America, Australia, New Zealand, and Southeast Asia) who will also be asked to change their secret question/answer in the next few days.

If you have not already, you should also consider getting yourself an authenticator.

UPDATE:  As previously indicated, the official blog has been updated with a post directing you to your account management page to change your secret question.