The Authenticator currently comes in a hardware form, costing $6.50 US, and is used to further secure players account in Blizzard's World of Warcraft. This would come in extremely handy for those looking to further secure their World of Warcraft accounts, but don't want an extra device (or have to pay $6.50) to do so.
There's no word on how far away the Mobile Authenticator is, or how much it will cost. You'll be able to use it for your Battle.net account also, for as many accounts as you desire. For the iPod touch iteration, you will need a WiFi connection to initially activate the Mobile Authenticator; after this, you won't need a network connection. Pricing is expected to be lower than $6.50, as that cost is mainly for hardware costs. Also, it would make sense Blizzard would want to keep pricing down as low as possible, in order to help as many people as possible keep their accounts secure.
Keep an eye on Neowin's gaming section to see when Blizzard plans to release the application via Apple's App Store.