Moments ago Redsn0w 0.9.6rc9 was released for iOS 4.3.1 Untethered Jailbreak, according to the iPhone Dev Team and i0n1c who originally found this exploit says, it works on iPhone 4, iPhone 3GS, iPad, Apple Tv 2G, iPod Touch and almost all iOS Devices but except iPad 2. So here is the step by step guide How to do Untethered Jailbreak on iOS 4.3.1
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Mirror for MAC
Open Redsn0w 0.9.8 and browse for the respected Firmware, which will be iOS 4.3.1.
When done proceed next and select install Cydia.
Now proceed next and switch of your Device. When done with, go next and follow the steps to enter into DFU mode.
Step 5: The process will start on your device and sit back and enjoy. You will be done in no time. When it will be done, it will say Done!.
That’s it now you are on Untethered iOS 4.3.1 Jailbroken Device!