Update #3: For those Mac users with an old-bootrom 3GS who really know what they’re doing, here’s a minimal 3GS 4.2.1 bundle that will get you to 4.2.1 without updating your baseband. Be sure to uncheck “Activate the iPhone” using Expert mode. To actually jailbreak after you’ve restored with the help of that bundle, please use redsn0w. If you don’t know how to drop a bundle into PwnageTool.app, please hold off on 4.2.1 until it’s untethered for everyone

See the Dev-Team’s full post HERE.

Update #2: The notion of a “tethered” jailbreak is pretty new to many people, so here’s a quick rundown on what to expect:

    1. If you’re on an iPhone3G, old-bootrom iPhone3GS, or non-MC ipt2g, life is easy. redsn0w installed an untethered jailbreak and so nothing below applies.
    2. “Tethered” does not mean you cannot boot at all without PC/Mac assistance. If you have not installed any tweaks that hook into important programs like SpringBoard or CommCenter, your device will actually boot. However, jailbreak programs like Cydia won’t work (and Cydia may still have a white icon). Also, certain built-in apps that had to be moved by Cydia will fail (Safari being the most noticeable example).
    3. If you’ve installed MobileSubstrate tweaks that hook into SpringBoard or other important programs, your boot will actually fail (you’ll get stuck at the Apple logo). You need to use redsn0w to “Just boot tethered right now”.

Remember, @comex and others are working on a way to untether the 4.2.1 jailbreak. Meanwhile, the above 3 points hopefully will make it all seem less confusing

Update #3: (Warning: if you use the ultrasn0w unlock, please read no further…this doesn’t apply to you yet!) We’ve made some updates to redsn0w to make it easier for jailbreak developers (and tinkerers) to get their programs ready for 4.2.1. As noted above, the public version of Cydia (and MobileSubstrate too!) is not 4.2.1-compatible. redsn0w will now let you install your own custom bundles independent of Cydia (the bundle can actually be Cydia if you’ve compiled it on your own). These bundles can be up to 15MB in size, and should be in the form of a gzip-compressed tar file.

The new redsn0w 0.9.6b3 is available at:

* OS X
* Windows

It’s very important that you get the file permissions and ownerships right in your custom redsn0w bundles. To give you a practical example of such a bundle, here’s one that includes OpenSSH, OpenSSL, and the basic apt installer programs:

* SSH bundle v2 (update: v2 has fixed permissions..you can just drop this one right in even if you installed the first version)

redsn0w has also been updated to recognize the 4.2.1GM IPSWs. *However*, as noted above, the 4.2.x jailbreak is not yet untethered for most devices! That means until someone like @comex comes up with a way to untether it, you must use redsn0w (or a similar utility) to boot your device into a jailbroken 4.2.1 state. (The only exceptions to this are the iPhone3G, non-MC iPod touch 2G, and old-bootrom iPhone3GS. redsn0w will jailbreak those untethered!)

With the above redsn0w and SSH bundle, jailbreak developers and tinkerers can jailbreak and SSH into their 4.2.1 devices, provided they’ve done a tethered boot (using redsn0w’s “Just boot tethered right now” option).

Note: The Cydia that’s included in 0.9.6b3 is the same one as in 0.9.6b2, and so it will *not* work on 4.2.1. Don’t try installing it on 4.2.1! Instead, use the SSH bundle, or compile Cydia on your own. If you’re familiar with the apt utilities, you can use “apt-get” to install many programs from the command line. Be sure to do “apt-get update” first to refresh your sources!

PLEASE CONSIDER THIS AN ADVANCED TOPIC!! It’s not meant for the masses because it involves rather nerdy things like command lines and tar files. But for those who know how to use this new redsn0w feature, have fun!

Update #2: By all accounts, we’re within a few days of Apple’s official public release of Firmware 4.2. Here’s what you need to know:

    • Thanks to geohot’s limera1n exploit, and our original pwnage2 exploit, and @pod2g’s ipod2g-MC exploit, absolutely all devices at all iOS firmware versions are capable of being jailbroken.
    • The untethered jailbreak of those very latest FWs and latest devices depends on @comex hacks. His hacks so far extend only to 4.1 and 4.2beta3. He’s working on a way to extend it to 4.2 and beyond. Just wait for him to work out his method.
    • iPhone 3G and 3GS unlockers will be covered by our upcoming unlock. Stay away from any updates to Apple FW until our official release and you’ll be okay. Just stay away from all Apple IPSWs
    • iPhone4 unlockers are not left out in the cold. @sherif_hashim has found some very promising avenues to pursue. Those will be explored as soon as possible after all the 4.2 madness.

What does this mean to you?

    • If you’re an unlocker, just stay where you are. Please, just stay where you are. Any mistakes you make now may be permanent.
    • If you only care about the jailbreak and you’re absolutely sure you have your personalized 4.1 SHSH hashes, feel free to experiment but keep in mind that any mistakes you make may result in your losing pictures or notes or bookmarks that you’d rather keep. Honestly unless you love living on the bleeding edge, it’s better to just wait for official updates from Cydia/redsn0w/PwnageTool.
    • Don’t buy or donate to any unlock or jailbreak scammers. Every legitimate solution you will find for unlocks or jailbreaks will be offered without an extended hand. That’s how the iPhone jailbreak/unlock community has succeeded. It’s about freedom to do what you want with your $300 device — not about donations, egos, tweets, or “interviews.”

Check out the full article HERE.