UPDate 2: Attention! We’ve received info, that Apple stopped signing iOS 4.2 for devices for which iOS 4.3 is available. This means you will not be able to restore to 4.2.1 without saved SHSH keys.
Using this tutorial you will be able to jailbreak iOS 4.2.1 running on iPhone 3G, iPhone 3GS,
We are not developers of jailbreak utilities, so we cannot assure that everything will work just fine. However we tried this tutorial on several devices and succeeded.
We will use iPhone 4 in our jailbreak tutorial. The process is the same for all devices.
Make a folder called “Pwnage” on the desktop. Download a couple of things: