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It looks like a bug in iOS 5 allows you to unlock iPhone 4S without the need to jailbreak it or using hardware methods such as GEVEY SIM.
The bug was discovered by Michael Capozzi. He managed to unlock his iPhone 4S to use it on T-Mobile USA's network but according to him the method should also work with any GSM carrier.
He has been kind enough to also give the following step-by-step guide to unlock iPhone 4S using the bug in iOS 5 to work on T-Mobile USA: