Ultrasn0w 1.2.1 carrier unlock now available for jailbroken iPhones running iOS 4.3.1
It's important to note that this isn't a new carrier unlock, and still requires an older baseband. Therefore if you're intending to upgrade to iOS 4.3.1 make sure you use something like Pwnage Tool to maintain your existing baseband, and do not just upgrade using iTunes in the hope of using redsn0w to jailbreak, as that will result in ultrasn0w being locked out.
Once you've jailbroken your iPhone using Pwnage Tool, fire up Cydia and search for ultrasn0w to get your carrier unlock installed, then just reboot your phone. Voila! Carrier unlocked.