Apple released iOS 4.3 alongside the iPad 2 and ever since it came out, hackers and developers have been working hard to produce jailbreak and unlock tools. iPhone hacker and developer iH8sn0w has managed to port a new tool called ultrasn0w fixer, which will unlock iPhone 3GS baseband 4.26.08 on iOS 4.3, which has previously been unlocked using iPad baseband 6.15.00.
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Ultrasn0w fixer was developed by msftguy, the developer of TinyUmbrella, when iOS 4.3 was in beta stages. iH8sn0w has been able to port it to the official iOS 4.3. iH8sn0w has also said that he will release a new version of Sn0wbreeze which will include ultrasn0w fixer, that will let you unlock iPhone 3GS baseband 4.26.08 on iOS 4.3
Remember that this works on iPhone 3GS only. Unlock for iPhone 4 is currently in the works. However, you will be able to unlock iPhone 4 on iOS 4.3 / 4.2.1 / 4.1 basebands 4.10.01 / 3.10.01 / 2.10.04 with the ultrasn0w fixer, once it is released for iPhone 4.
iH8sn0w has announced via Twitter that he is indeed going to release the new version of Sn0wbreeze which will include ultrasn0w fixer for iPhone 3GS, tomorrow. No exact date for iPhone 4 ultrasn0w fixer has been given.
In the meantime:
Learn How To Jailbreak iOS 4.3 Tethered Using Sn0wbreeze
Download iOS 4.3 for iPhone, iPad and iPod touch.