As promised, the iPhone Dev Team released an update to the UltraSn0w unlock for iOS 5 today. This update allows users with unlockable basebands running iOS 5 to unlock their iPhone 3GS, or iPhone 4.
This version of UltraSn0w (1.2.4) doesn't add any new basebands to the compatibility list, but if you're running basebands 4.26.08, 05.11.07, 05.12.01, 05.13.04, or 06.15.00 on the iPhone 3GS, or baseband 01.59.00 on the iPhone 4, then you can unlock your device right now...