Blizzard hasannouncedtoday that they have reason to believe that hackers have infiltrated their systems and stolen sensitive data. The extent of the leak is not completely clear, however they said that they believe its possible that personal data was taken, including encrypted credit card information.
They have said, however, that the credit card information is encrypted in a such a way that should make it fairly difficult to retrieve, as each card would have to be decyphered individually.
Still, they are asking that users take steps to protect themselves, and their first suggestion is changing your Battle.net password. The real danger is that the thieves could gain access to potentially lucrative Diablo III accounts. Or they could log in to World of Warcraft and force you to buy dozens of vanity pets. The horror.