Blizzard have released a special statement regarding all Battle.net accounts stating that there was some sort of hack or "unauthorized illegal access" to a large quantity of US Battle.net accounts. According to Blizzard no credit card or billing address information was obtained, but email addresses, encrypted passwords, and security questions were all stolen. While the chance of the information actually being utilized is slim, Blizzard recommends you change your password immediately.
For anyone playing Diablo 3, Starcraft 2, or World of Warcraft - your account is affected. Even if you use a Mobile Authenticator for your account, there seems to be some sort of issue. Blizzard will release an updated version of the software for these devices.
Follow this link to change your password.
Source: Us.blizzard.com