The Gevey SIM supports an unlock for the iPhone 4 on Apple's latest firmware, iOS 4.3.4. Apple recently pushed the 4.3.4 update to combat the exploit uncovered by the jailbreak tool, JailbreakMe, in iOS 4.3.3.
If you upgrade to iOS 4.3.4, you lose your untethered jailbreak. But the Gevey SIM still works...