Flashing the iPad Baseband and using UltraSn0w, which is a software unlock that requires that your device be jailbroken. Since your device is a 3GS, and F0recast says that it's unlockable via iPad BB, that means you have the old bootrom 3GS. If you'd like to go with the unlock on iOS 5.1.1, unlock it as is. Or, if you want to upgrade to iOS 6.1, it'll work as well.