SAM (Subscriber Artifical Module) is an carrier lock tweak for any iPhone that is iOS 5+.

I have tried this app personally and it works great (I got unlocked from Vodafone on 4S)

If you wish to unlock the repo is

It is a bit complex but you can follow the official how-to-guide on using this

In case you are already having doubts with this great news, MuscleNerd was actually able to confirm this unlock works as he unlocked his T-Mobile iPhone 4S with it. Meaning this is indeed a legit software unlocking solution

*Thanks to iJailbreak for telling me about this and the pics