According to a RedmondPie report, The Gevey SIM Interposer is reportedly confirmed working to unlock the iPhone 4 in the U.S.
Gevey recently released a SIM Interposer which can unlock the iPhone up to iOS 4.3 by force activating the baseband using the emergency dialer.
The SIM Interposer works by adding a EEPROM chip to your SIM card just like a TurboSIM. You then call 112, a number that can be called from any GSM phone, free of charge, with or without a SIM card on any compliant network. Once the call connects you hang up then toggle Airport mode on and off. When the network connection resumes you will be unlocked.
This method of unlocking is not recommended due to the legality of dialing 112 in certain countries. You can learn more about how the unlock works here.
