The iPhone Dev Team has updated the UltraSn0w unlock tool with support for iOS 4.3.2. You can use PwnageTool to jailbreak your iPhone and preserve your baseband for the latest UltraSn0w unlock.
The latest release of UltraSn0w does not support new basebands, it only fixes compatibility issues with already-unlocked iPhones. In this tutorial, we'll show you how to install the latest version of UltraSn0w to unlock your iPhone running iOS 4.3.2.