After the Dev-Team had released the untethered jailbreak for iOS 4.3.1 on all iOS devices from iPhone to iPad(except iPad 2) their next objective was to update Ultrasn0w to work on iOS 4.3.1. They have finally achieved it, and after final testing Ultrasn0w 1.2.1 is now live in Cydia for us to download.
If you own an iPhone 4 or iPhone 3G/3GS that is not factory unlocked, you can install Ultrasn0w to use it with any carrier. The only requirement is an iPhone running jailbroken iOS 4.3.1.
iPhone 4 users must take extra care while updating to iOS 4.3.1 as they need to prevent the baseband from upgrading. This update only brings iOS 4.3.1 support to basebands which could be unlocked earlier. So even now, only baseband version 01.59.00 can be unlocked on iPhone 4.