After a long expectation, the Dev Team just released an updated version of UltraSn0w that will unlock the iPhone 4 as well as the iPhone 3G and 3GS running iOS 4.0.1.
After the JailbreakMe jailbreak released this weekend, the UltraSn0w 1.0-1 carrier unlock is the cherry on top. It is compatible with the iPhone 4 baseband 01.59.00 and the iPhone 3G/3GS basebands 04.26.08, 05.11.07, 05.12.01 and 05.13.04.
You should see UltraSn0w in Cydia under the repo666.ultrasn0w.com repo.
Good job, Dev Team! Good job.