I can imagine the pain that iPhone 4 owners are having to go through just because they updated their basebands to 3.10.01, 2.10.04, 4.10.01 and 1.59.00 as no software unlock is available for these basebands. To unlock these higher basebands, solutions like the Rebel SIM and Gevey SIM interposers are used which is not a permanent solution, not to mention the fact that their legality is still an issue. However, the makes of Gevey SIM are now releasing Gevey Ultra to unlock iOS 5, iOS 4.3.x iPhone 4 on basebands 3.10.01, 2.10.04, 4.10.01 and 1.59.00.
Why is Gevey SIM's legality an issue? Basically, it forces baseband activation through the emergency dialer. The procedure works if your network handles 112 calls properly according to the GSM standard and that they are tolerant to TSMI spoofing and do not actively validate your SIM again for incoming calls. Calling the 112 emergency number and hanging up is considered illegal in USA and other countries.
Gevey Ultra works through a different procedure that requires a jailbreak for iPhone 4. Gevey SIM and Gevey Pro did not need a jailbreak as a pre-requisite for iPhone 4 unlock on basebands 2.10.04, 3.10.01, 4.10.01 and 1.59.00. To use Gevey Ultra for an iPhone 4 unlock, you first need to jailbreak your iPhone 4, then launch Cydia and add this repository: cydia.furiousMod.com and install the FuriousMod package from the repository that you just added. When your iPhone 4 re-springs, insert the SIM card along with Gevey Ultra and just wait for the signals. After about 3 minutes or so, your iPhone 4 will be running on the carrier of your choice, regardless of the higher baseband. Also, you won't have to repeat the procedure again to unlock using Gevey if you reboot your iPhone 4 or it runs out of charge. Gevey Ultra is going to be devoid of such problems, which have existed in previous variants of Gevey interposer.
On another note, if you don't want to unlock your iPhone 4 through software or Gevey interposer hardware, official unlocked iPhone 4 are now being sold at Apple Retail Stores in the USA. Nevertheless, you can watch the video guide of how to unlock iOS 5, 4.3.x iPhone 4 on higher basebands using Gevey Ultra embedded below.