Blizzard has released an iPhone and iPod Touch application that works as an authenticator to help keep your World of Warcraft and Battle.net accounts secure.
The Battle.net Mobile Authenticator is only available in the US at the moment - perhaps because it requires merged Battle.net and WOW accounts, something that's not yet possible in Europe and Asia.
Blizzard says that versions for other mobile phones are also in the works for the future.
It works just like the existing keychain authenticator, generating a one-time security code you'll need to enter each time you log in. However, unlike the keychain authenticator, the mobile app is free of charge.
You can read more about the Battle.net Mobile Authenticator at the official FAQ.