Here it is, folks; the much anticipated iOS 4.2.1 untethered jailbreak. It’s not from the iPhone Dev Team, but from the other renowned group composing the iPhone jailbreak front, the Chronic Dev Team. GreenPois0n RC5 is out now, and unlike the latest version of redsn0w, this one's capable of jailbreaking most iOS device on iOS 4.2.1 right out of the box even without SHSH blobs of 4.2b3 saved on Cydia.
Got your attention, yet? If so, here’s how to go about it: first, download GreenPois0n to your Mac (sorry Windows/Linux users, this one's currently for Mac only). Next, connect your iPhone to your Mac, launch GreenPosi0n, and then hit the Jailbreak button. Now, put our iPhone into DFU mode. After doing this, Greenpois0n will now start jailbreaking your iPhone. That's it. Wait for it to finish, and you're done. You've now got yourself a jailbroken iPhone on iOS 4.2.1.
To install Cydia, simply launch the Loader app installed on your jailbroken iPhone.
Not clear enough? Here’s a step-by-step video guide courtesy Redmondpie:
Just a reminder, if you rely on an iPhone unlock, don’t upgrade to iOS 4.2.1 as there’s still no unlock available for the new baseband that’s bundled with this iOS version.
If not, feel free to go ahead jailbreak your iPhone (at your own risk, of course).