Updated with new solution
To unlock your iPhone 4 on iOS 5 with Gevey SIM you MUST update your iPhone 4 to pre-jailbroken custom firmware to preserve the unlockable baseband which are compatible with Gevey SIM.
A new Cydia package called “SemiTether” jailbreak for iOS 5 is now available on BigBoss repository which basically helps reboot tethered-jailbroken iPhone, iPad and iPod touch without having to plug in your device into PC or Mac.
[We Test it And Worked Fine ]
The Gevey team introduces Gevey SIM Interposer, a small chip to unlock iPhone 4 on 02.10.04,03.10.01 Baseband and iOS 4.1, 4.2.1 , 4.3, 4.3.1, 4.3.2, 4.3.3, 4.3.4, 4.3.5 . This reminds me of the good old TurboSIM days, a popular method for unlocking iPhone 2G in 2008.