The best part? This jailbreak does NOT rely on your SHSH blobs. That means you can jailbreak your new devices as well, right out of the box!!!
I successfully ran Greenpois0n RC5 on my 3GS over the top of my 4.2.1 tethered Redsn0w 0.9.6rc8 jailbreak. No issues what so ever. I no longer need my laptop or PC to reboot my phone. Thank you Chronic Dev Team!!!!
On a side note, it does take a while for your iDevice to reboot after the initial jailbreak. No worries. Once you get to the rotating Apple/Greenpois0n logo, you are good to go.
So if you are are currently on the Redsn0w 0.9.6 jailbreak, go download and run Greenpois0n RC5.
Here is the download link for Greenpois0n RC5
Here is a great site from comex & chpwn, JAILBREAK QA. Go here for questions and answers about jailbreaking your iDevice.
Here is a tutorial video posted on YouTube by Xadget.
Please follow these tips BEFORE you jailbreak:
1. If you are already jailbroken, use PKGbackup or some other back up app in Cydia to back up your Cydia apps. I prefer PKGbackup [$7.99] because it backs up your settings and preferences, APT does not.
2. Transfer purchases & backup your iDevice in iTunes.
3. If you are using 4.2.1 onRedsnow0.9.6, rung Greenpois0n and you will be untethered, your will not lose your data or apps.
4. If you are currently Jailbroken on 4.1, upgrade your firmware to 4.2.1 using iTunes. You will have to start from a fresh jailbreak and restore from backup.
5. If you used PKG backup[or some other app], install it from Cydia after your new jailbreak and use it to restore your Cydai apps.
Enjoy your Jailbreak. Be sure to comment on how Greenpois0n worked for you.
And that's how Bruce T does it.