Following the release of the updated PwnageTool 4.3.2 jailbreak tool, the iPhone Dev Team has also pushed out an update to the UltraSn0w unlock in Cydia.
UltraSn0w 1.2.2 has been released to support the new iOS 4.3.2 firmware, as the previous version of UltraSn0w (1.2.1) was broken in iOS 4.3.2. Please note, this will only work for the older iPhone 4 baseband 1.59.00, and iPhone 3G/3GS basebands 4.26.08, 5.11.07, 5.12.01, 5.13.04, and 6.15.00. It can not unlock newer basebands since there is currently no unlock available.