UPDATE 1: Windows version has been released, however it is still in beta so there could be slight bugs.
Greenpois0n RC5 has been released, the chronic dev team announced a few weeks ago that 4.3 had closed the loophole they had been using to jailbreak. So they decided to release it for 4.2.1, the jaibreak works on all devices, and is untethered.
There has been slight issues when jailbreaking, but just keep trying and it should go fine.
- iPod Touch 2G (all bootroms)
- iPod Touch 3G
- iPod Touch 4G
- iPad 1G
- iPhone 3Gs (all bootroms)
- iPhone 4
- iPhone 4 Verizon (_not_tested_)
- AppleTV 2
- plug the device to your computer using the USB cable
- quit iTunes if it shows up
- start greenpois0n.app
- follow instructions
- the device will boot a 1st time with verbose text on screen. This operation will install the jailbreak on the device.
- the device will reboot normally
- start the Loader application (green icon on the SpringBoard), and follow instructions to install Cydia
However this has only been released for the Mac OSX so far, and windows and linux versions will be released later.