I'll admit that I've never used a Gevey SIM interposer, or anything like it in the past, but I figured I'd give it a shot with the iPhone 4S.
The iPhone 4S has its own special SIM interposer called the R-SIM II Ultra S, which was recently released.
I'm happy to report that this is by far the easiest method for "unlocking" your iPhone 4S, and using it with T-Mobile or virtually any other GSM carrier. And guess what, no jailbreak is required.
Check out our review of the iPhone 4S R-SIM tool inside...