iH8sn0w has announced the release of Sn0wbreeze 2.3b4 which is the new updated version of Sn0wbreeze 2.3. It comes with support for Ultrasn0w iOS 4.3 unlock. Sn0wbreeze 2.3b4 patches ultras0w to unlock iPhone 4, 3GS on iOS 4.3. Now you can upgrade your iPhone 3GS to baseband 06.15.00 and unlock your iPhone running iOS 4.3 using ultrasn0w. After upgrading your iPhone 3GS baseband to 06.15.00 you may lose your GPS. Earlier iH8sn0w has posted a teaser video which shows the iPhone 3GS unlock running iOS 4.3 using Ultrasn0w.
Sn0wbreeze 2.3b4 supports Ultrasn0w unlock for basebands 01.59.00, 04.26.08, 05.11.07, 05.12.01, 05.13.04, 06.15.00. You should note that Sn0wbreeze 2.3b4 is not compatible with basebands (02.10.04 / 03.10.01 / 04.10.01 / 05.14.02 / 05.15.04). You can download Sn0wbreeze 2.3b4 to Jailbreak and Unlock iOS 4.3 with Ultrasn0w from here.