With the latest Apple firmware for the iPhone now official Dev-Team has started to talk about how to unlock your Apple device once the update is applied and with devices that are already unlocked. Naturally, if you are unlocked already, you should NOT apply the new official update, as this will clamp your iPhone down again. Dev-Team reminds that already unlocked users need to wait for the revised update before applying.
If your device has not unlocked and has the new update already, you can follow some steps that are offered on the Dev-Team blog. Those using the older iPhone 3G and 3GS will be able to do an untethered jailbreak with 4.2.1. However, owners of the iPhone 4 with the new OS will have to do a tethered jailbreak. That means each time your iPhone loses power or is turned off you will need to connect it to a computer to boot into the jailbroken state.
Dev-Tem says that @comex is working on an untethered jailbreak for the 4.2.1 firmware and asks that you not harass him about when it will be complete. What is known about the @comex jailbreak is that you will have to use the 4.1 SHSH blob to use it.