Gevey SIM for iPhone 4 basically works by forcing the activation of the baseband using the emergency dialer, and it is also said to be vulnerable up to iOS 4.3. You can read more about how it actually works by navigating to this link here.
for more information go to Gevey SIM Can Unlock iPhone 4 in US On Baseband 2.10.04 / 3.10.01 Proved ? Unlock iPhone
for more information go to Gevey SIM Can Unlock iPhone 4 in US On Baseband 2.10.04 / 3.10.01 Proved ? Unlock iPhone