Applenberry, the manufacturers of Gevey SIM are now releasing Gevey Ultra to unlock iPhone 4 on baseband 2.10.04, 3.10.01, 4.10.01 and 1.59.00. Unlike previous Gevey interposers, Gevey Ultra requires a jailbreak for unlocking iPhone 4 using Gevey Ultra.
Previously, when an iPhone was unlocked using Gevey SIM, the whole unlocking procedure had to be repeated over again if the iPhone 4 was rebooted or if it ran out of battery. The Gevey Ultra won't have this issue, and you wouldn't have to repeat the whole procedure once again if your iPhone 4 rebooted. Alternatively, you can also purchase an officially unlocked GSM iPhone 4 from Apple.
With previous models of Gevey interposer, no jailbreak was necessary. However, Gevey Ultra requires your iPhone 4 to be jailbroken before you can unlock it. So you must jailbreak your iPhone 4 first, then add this repo in Cydia: http://cydia.furiousMod.com and then install the FuriousMod package. The iPhone 4 will prompt you to restart SpringBoard, after that simply insert the SIM in to the iPhone 4 along with Gevey Ultra and wait your the device to catch signals. Within about 5 minutes, your iPhone 4 will be fully unlocked on basebands 2.10.04, 3.10.01, 4.10.01 and 1.59.00 running iOS 5 beta, iOS 4.3.3 and older firmwares.
Gevey interposer unlocking solution is considering illegal in many countries including the US because it dials the 112 emergency number and then hangs up. Gevey Ultra does not require this, so the legal status of this is yet to be decided.