Valve has opened a beta test for two factor authentication login on its Android and iOS apps. The Steam Guard Mobile Authenticator works similar to Blizzards Battle.net authenticator in that it will provide codes to a user to input while logging in. Of course, this could potentially mean it has the same problems as the Battle.net authenticator.
Steam generally takes a cautious approach to security, with multiple layers already in place to prevent users from being hacked and having their inventory stolen. At the moment, Steam Guard already has two factor authentication in place. It currently requires users to input a security code sent by email when logging into an account from a new location. The Steam Guard Mobile Authenticator looks like it may replace the security email, which has had its critics for being slow and not promptly delivering the passcode.
The system is currently in a limited beta test, and interested users will have to join the related Steam Community Group in order to participate. The Steam Android and iOS apps should be getting updates to implement the feature soon, but only those participating in the beta will be able to use it before the option is officially released. Bear in mind that Valve tends to take its time with beta tests.
Valve says that the Android app update has been pushed out, and should have received it by now. On the other hand, the feature will be tested on the iOS app when it is made ready. That being said, the Steam Community post claims that the iOS version should be available soon.