iPhone Dev Team has released Redsn0w 0.9.6 Beta 3 to support iOS 4.2.1 GM IPSW.It lets you to install custom iOS 4.2.1 firmware bundles independent of Cydia.These bundles can be up to 15MB in size and should be in the form of a zip-compressed tar file.
Warning: iPhone unlockers should stay away from Redsn0w 0.9.6 Beta 3. Also, this update is only for advanced users and jailbreak app developers who want to get their programs ready for iOS 4.2.1.
Redsn0w 0.9.6 Beta 3 is a tethered jailbreak for iPhone 4, iPhone 3GS (new bootrom), iPod Touch 4G, iPod Touch 3G, iPod touch 2G (MC model), and iPad. Which means you must must use redsn0w (or a similar utility) to boot your device into a jailbroken 4.2.1 state.
Only iPhone 3G, iPod touch 2G (MB), and iPhone 3GS (old bootrom) users can have an untethered jailbreak.
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