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iOS 4.3.1 released yesterday has already been jailbroken using PwnageTool 4.2 for Mac that iPhone dev team had released to jailbreak iOS 4.2.1.
This has been possible thanks to the iOS 4.3.1 custom bundles for iPhone 4, iPhone 3GS, iPod touch 4G, iPod Touch 3G, iPad and Universal Ramdisk fixer released by DjayD6.
The iOS 4.3.1 custom bundles need to be used along with PwnageTool 4.2, Universal Ramdisk Fixes and tetheredboot utility to jailbeak iOS 4.3.1.