As you have no doubt heard by now, iOS 4.2.1 is available for iPhone, iPod Touch and iPad users. In preparation for jailbreaking, you will want to back up your SHSH blobs. That is easy with TinyUmbrella, which allows you to do so.
Here's a quick guide to get you started:
1) Download the software here
2) Connect your device via your Apple USB cable.
3) Click on save SHSH (shown below)
4) You will see a progress bar.
5) You are now done, and have saved your SHSH blobs, this is the first step in preparation for jailbreaking.