The jailbreak method is a little involving, but if you’re willing to give it a shot, the whole process should take less than 15 minutes. Also note that if you’re looking for an untethered jailbreak method, you will need to wait until a new version of limera1n or greenpois0n is released – as of now, only tethered jailbreaks work with iOS 4.3.1.
The jailbreak uses the same method used with previous versions of iOS, as it requires PwnageTool 4.2, and additional packages compatible with iOS 4.3.1. A full walk-through is already available at Redmond Pie
Also note that you may want to save your SHSH blobs before jailbreaking your device. Apps like TinyUmbrella
will do it for you (the tool was updated to support iOS 4.3.1 earlier today).