There was a time when DevTeam released an update to ultrasn0w which could unlock iPhones on all BaseBands. Fortunately, that day hasn’t come yet again. It was one of those memorable days in the history of iPhone unlocking. Several method have appeared since iPhone unlocking scene has come into action. Some of those are hardware related and some of them are software based. However, one should have a bit of understanding on which method to use for the respective iPhone model.
Hardware Solutions:
Here, i want to summarize all the ways possible at the moment. SAM method is now permanently closed and the only way to make it work on iPhone 4 or iPhone 4S on iOS 5.1.1 is through saved validation tickets on a respective micro-sim. Fortunately, latest version of Redsn0w provides you the option to activate it with ease. Another method to unlock iPhone 4 or iPhone 4S is through Gevey Sims which even works now on both the devices. It’s a sort of sim interposer which is a replacement of the existing sim tray interposed with an IMEI spoofer. 
Let’s not go into much technical details and talk about the various sim interposers sold in the markets. For iPhone 4, you could use the normal Gevey Sim which requires you to dial 112 (illegal in some countries) for the unlock. On the other hand, iPhone 4S could be unlocked without the need of dialing 112 using Gevey Ultra S. It’s similar to ‘plug and play’ where you just have to insert the sim interposer tool and the iPhone 4S is unlocked. Gevey Ultra S is also works for iPhone 4, in case if normal Gevey fails.
Software Solutions:
For iPhone 3G or iPhone 3GS, update to the latest firmware, install the iPad BaseBand and unlock using ultrasn0w via Cydia. You could do with the help of popular tool called Redsn0w. Follow these steps only if you are on a BaseBand which is unlockable. Otherwise, you could anytime downgrade back to 5.13 by using the same tool. Another way is to preserve BaseBand after restoring with the custom firmware.
Whereas, iPhone 4 can’t be unlocked via ultrasn0w on a BaseBand which is greater than 1.59.00. If you had been preserving BaseBand from a quite long time, keep doing it until a successive BaseBand is supported. The probability is certainly less, but who knows one day some exploit is found and it becomes possible. On the flip side, the iPhone 4S as of now is not unlockable through any software solution which means that SAM unlock was the first one ever.
Nevertheless, DevTeam had been working for an iPhone 4S unlock where they dumped the BaseBand to trigger the procedure. But we suspect that the process has turned out to be a failure due to which the progress was never notified through DevTeam’s twitter account.
Anyways, this is the current state of iPhone 3GS, iPhone 4 & iPhone 4S unlock. We hope you would have understood about which method to follow and what measures you need to take before upgrading your respective iPhone. If you are still in doubt, our comment section is always open for you. We will try to reply back shortly.