Blizzard is reporting a recent security breach, where some data was extracted from their internal network. They’re recommending that you change your password and personal security question just to be safe.
Some data was illegally accessed, including a list of email addresses for global Battle.net users, outside of China. For players on North American servers (which generally includes players from North America, Latin America, Australia, New Zealand, and Southeast Asia) the answer to the personal security question, and information relating to Mobile and Dial-In Authenticators were also accessed. Based on what we currently know, this information alone is NOT enough for anyone to gain access to Battle.net accounts.
Head on over to the Blizzard website for complete details.