A jailbreak of iOS 4.3.1 is now possible with both PwnageTool on Mac, and Sn0wbreeze on Windows. RedmondPie -- who else? -- has some handy guides that you can follow for all iOS 4.3.1 devices on Windows, for iPhone 4 on OS X, iPad 1 on OS X, iPhone 3GS on OS X, and iPod touch 4G and 3G on OS X.
All iOS 4.3.1 jailbreaks are still tethered, meaning you'll have to jailbreak your device after every reboot. An untethered jailbreak is slated for release sometime in the next week -- but 4.3 was meant to have an untether, too, and that never emerged.
Maybe Apple's updated security mechanisms will finally keep hackers at bay!