Greenpois0n RC5 now allows you to jailbreak iOS 4.2.1 untethered on most iDevices such as the iPhone 4 and iPad. You won’t need any SHSH blobs saved like some other jailbreaks. The jailbreak for iOS 4.2.1 with greenpois0n is simple and to the point. Only bad thing is that it is mac only at the moment.