From the email they sent me:
If you have already purchased Diablo III or StarCraft II -- or are considering purchasing these games in the future -- and wish to protect your Battle.net account from unauthorized access when logging in to these games, we recommend one of the following:
- Battle.net Authenticator - a physical device that you can carry on your keychain, available from the Blizzard Store
- Battle.net Mobile Authenticator - a mobile app available as a free download for iPhone, iPod touch, and Android