The jailbreak for 4.2.1 has been out for a while, and for most of you, its whats know as a “tethered” jailbreak. Meaning you need a tool to boot the jailbroken device back up overtime it loses power or needs a restart. There are a couple of ways to accomplish this, redsn0w or if you made your own custom firmware with a modified pwnage tool, tethered Boot tool and terminal/cmd.
Now theres another way. iBooty from iH8sn0w. We’re not entirely sure if or when this will be made public, but the tool is basically a 1 button boot tool. You just need to add a file to your device via AFC2, SSH, or iFile, then run iBooty whenever you need to boot into the jailbroken kernel. It supports multiple devices, and there is no need to look for firmware first to use it. Good for on the move or portable booting from a computer other than your own.
FSM EXCLUSIVE DEAL At the moment its a Windows only tool, and as far as we know its going to stay that way. Mac users will need to continue using redsn0w or the tetheredBoot tool.
Hopefully soon, the untethered jailbreak exploit will be made public, but until then….