The Chronic Dev-Team has released Greenpois0n for Windows which brings an untethered jailbreak to iOS 4.2.1 devices. You can jailbreak iPhone 4, iPhone 3GS, iPod Touch 2G, iPod Touch 3G, iPod Touch 4G, iPad.
You can download the latest version of GreenPois0n from here.
The Chronic Dev-Team has recently announced via twitter that they will release their Greenpois0n untethered jailbreak for Windows this weekend.
Site issues should be better soon thanks to @iOPK. Windows version should be out tomorrow or the next day
Greenpois0n for Mac OS X with support of iOS 4.2.1 untethered jailbreak was released yesterday.
These are step-by-step instructions on how to jailbreak iOS 4.2.1 on iPhone 3GS, iPhone 4, iPad, iPod Touch 2G, iPod Touch 3G, iPod Touch 4G using Greenpois0n for Mac OS X. Tutorial for Windows users is available here. All the steps are the same for all devices.
Greenpois0n has been updated to support the iOS 4.1 and iOS 4.2.1 firmware. Use Limera1n if you want to jailbreak iOS 4.0, iOS 4.0.1, iOS 4.0.2.
Remember: Greenpois0n recommends a clean restore before continuing.
Unlockers: do not update to iOS 4.1 or iOS 4.2.1 if you need the unlock. Ultrasn0w does not support that baseband yet. The only exception is the iPhone 4. You can unlock iPhone 4 with iOS 4.1 using ultrasn0w. Experienced users can also try update to iOS 4.2.1 without upgrading your baseband by using TinyUmbrella. Start TSS Server->Start iTunes->Plug in your iPhone->Restore 4.2.1->Error 1013->Put phone into DFU Mode->Launch Greenpois0n.
We are not authors of this app. We do not promise that everything will go well, use this tutorial at your own risk.
The Chronic Dev-Team has finally released the new version of Greenpois0n, which features a full untethered jailbreak for iOS 4.2.1. No SHSH blobs needed. The exploit used has already been patched in iOS 4.3, so it not work on future iOS 4.3.
You can jailbreak iPhone 4, iPhone 3GS, iPod Touch 2G, iPod Touch 3G, iPod Touch 4G, iPad and Apple TV 2G. Unlike redsn0w 0.9.6 this is untethered, so there is no need to connect iOS device to a computer after restart.
There is only Mac OS X version available. Chronic Dev-Team will release Windows and probably Linux version soon.
Unlockers: do not update to stock iOS 4.2.1. You should keep your baseband version. Wait for new version of PwnageTool.
You can download GreenPois0n RC5 from here.