As promised, iPhone Dev-Team has just released Redsn0w 0.9.6RC9 for Windows and Mac OS X users. You can use Redsn0w 0.9.6RC9 to jailbreak iOS 4.3.1 untethered on every supported iDevice except Apple TV 2G. The untethered exploit used in Redsn0w 0.9.6RC9 is originally discovered by a German security researcher named Stefan Esser.
As Apple TV 2G is not supported by Redsn0w 0.9.6RC9, so you can always use Sn0wbreeze 2.5 on Windows and PwnageTool 4.3 on Mac to jailbreak iOS 4.3 on Apple TV 2G untethered. If you rely on an unlocked iPhone, do not use Redsn0w otherwise it will upgrade your baseband to 05.16.02 on iPhone 3GS and 04.10.01 on iPhone 4. Ultrasn0w 1.2 is incapable of unlocking either of these basebands, so Redsn0w is the best jailbreak option for those who do not rely on an unlocked iPhone.
Update#1: Jailbreak iOS 4.3.1 with Redsn0w 0.9.6RC9 on Windows & Mac [How-To Guide]
Update#2: Download Ultrasn0w Fixer to Unlock iPhone 4/3GS on iOS 4.3.1 [Unofficial]
Update#3: Download Redsn0w 0.9.6RC10 & Redsn0w 0.9.6RC11 for Windows & Mac OS X
Update#4: Download Redsn0w 0.9.6RC12 to Fix Boot Logos Lingering Issues
Download iTunes 10.2.1 for Windows & Mac OS X
Download stock iOS 4.3.1 IPSW for iPhone 4/3GS, iPod Touch 4G/3G, iPad 2/1st-Gen
Download Redsn0w 0.9.6RC9 for Windows [MediaFire Mirror]
Download Redsn0w 0.9.6RC9 for Mac OS X [MediaFire Mirror]