The Gevey SIM is sort of a hardware hack for the iPhone 4 which allows it to be used with any carrier in world, it supports quite a few basebands as well. The SIM is basically a SIM holder quite thinner than a normal SIM, you have to insert the SIM you want to use into the Gevey SIM and insert it into your iPhone 4.
The hack is now suppose to work on Apple’s latest platform for mobile devices: iOS 5.
Source: Redmondpie