This untethered jailbreak for iOS 4.3.1 is based on an exploit found by iPhone security expert and hacker Stefan Esser, who demoed it on his iPod touch last week before handing it over to the Dev-Team to do the rest.
iOS 4.3.1 was originally jailbroken last week using PwnageTool bundles (on Mac) and Sn0wbreeze (on Windows) but it was limited to tethered boot.
But now with this new version of Redsn0w and PwnageTool, you can jailbreak on the above mentioned devices, fully untethered on iOS 4.3.1.