For those of you who want to try this now - I don't recommend it, it's a pain, I say wait for the official jailbreak tools, but for those of you asking, here's a rough guide, please ASK QUESTIONS! if you need to! Myself, Farbod, and many others will try and help as much as we can.
The current version of
PwnageTool 4.1.2 will jailbreak iOS 4.2.1. Keep in mind you'll run just like you would any other version of PwnageTool. Our walkthrough can be found
here
Everytime you want to run Cydia, you'll need to reboot your device into this semi-untethered jailbroken state. If you absolutely can't wait, use either of these methods at your own risk. I still say wait for the official tools, which we know will release sooner rather than later.
Redsn0w 0.9.6b3 will jailbreak iOS 4.2.1 but there are a couple caveats and issues jailbreakers need to be aware of. First off, even though this jailbreak will work with any model iOS device running iOS 4.2.1, the version of Cydia packaged with it doesn't work. So you'll need to manually patch Cydia in order for it to work correctly.
After jailbreaking your device with Redsn0w, you'll need to to run it again and choose to install a custom bundle as choose to boot as tethered for now. Let Redsn0w load and then click next when the option becomes available and then you'll be required to enter DFU mode.
After your device is done installing, it should reboot. You should now be able to use any SSH client in order to access your device and install files. You will need to redo this process every time you need to SSH into your phone.
After this is done, you'll need to
download the patched Cydia file and install it via SSH to your phone.
This is the directory you need to install it to: /Applications/Cydia.app/Cydia_ directory
You will then need to type the following commands (in order) in Terminal:
* ssh root@<your device's IP address>
* Enter your root password, by default this is 'alpine'
* apt-get update
* apt-get install bsdiff
* cd/Applications/Cydia.app/
* cp Cydia_ Cydia.bak
* bspatch Cydia.bak Cydia_ Cydia.patch
* chmod +x+s Cydia_
Both of these methods are somewhat of a pain, I still say wait for the official tools. There's also no guarantee all your jailbroken programs and apps will work under 4.2.1 either.
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Jailbreak iOS 4.2.1 via Redsn0w 0.9.6b3 or PwnageTool 4.1.2 
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