If you are one of those folks who rely on ultrasn0w or Gevey Sim, you can download the custom firmware for your iPhone. It will allow you to preserve BaseBand of your iPhone on iOS 5 and use the previous solution again. iOS 5 custom firmware is now possible through Redsn0w 0.9.9b7, available for both Windows and Mac.
If you have accidentally updated your iPhone 4 or 3GS to iOS 5, you can’t revert back your BaseBand through any method. Hence, you won’t be able to unlock your iPhone either using Gevey Sim or ultrasn0w until they are updated in the future. Undoubtedly, you can make custom firmware by yourself. But if you want to download the ready-made custom ipsw for iPhone 4 or iPhone 3GS, you can download them from here:
Once downloaded, use the DFU mode option from Redsn0w, select the ipsw using Shift+restore (Windows) and Option+restore (Mac) in iTunes. If you face any error during the restoring process, let’s know in the comment section. We will definitely try to help you.