iH8sn0w has just released Sn0wbreeze 2.3 Beta 4 jailbreak for iOS 4.3 that has come up with a fix for Ultrasn0w. The Ultrasn0w fixer has been demoed on video by iH8sn0w showing to be working on an iPhone 3GS with baseband 4.26.08 running on iOS 4.3.
Ultrasn0w fixer was developed by msft_guy in beta stages of iOS 4.3 and then was being made open source. The process used currently can only be used for iPhone 3GS on baseband 4.26.08. iH8sn0w is working on it to work for iPhone 4.
iH8sn0w has also confirmed that 3GS users with iPad baseband will soon be able to use iOS 4.3 with Ultrasn0w. He has also tweeted that the its currently under testing phase and it will be released for public probably tomorrow.
Checkout the video below.