Ultrasn0w is a tool created by the iPhone Dev Team, which serves as a Carrier Unlock for all jailbroken iPhones. The current supported baseband for this unlock tool is 06.15.43. Apple has found the Limera1n exploit and patched it on May 23, 2011. All users of Ultrasn0w will be affected as the baseband will now be updated to 06.17.56, rendering all other carriers unable to provide service to these iPhones.
The iPhone Dev Team is said to be waiting to release an updated version to compensate for the updated baseband.