The iPhone dev team released another edition of both PwnageTool and redsn0w to bring an untethered jailbreak for Apple's latest firmware 4.3.1.
The 4.3.1 untether exploit works on all devices that actually support 4.3.1 except for the iPad2:
iPod touch 3G
iPod touch 4G
AppleTV 2G (PwnageTool only for now)
The reason the untether won't work as-is on the iPad2 is that it requires a bootrom or iBoot-level exploit to install, and the iPad2 is not susceptible to either the limera1n or SHAtter bootrom exploits.
It's important to note that this isn't a new carrier unlock, and still requires an older baseband. Therefore if you're intending to upgrade to iOS 4.3.1 make sure you use something like Pwnage Tool to maintain your existing baseband, and do not just upgrade using iTunes in the hope of using redsn0w to jailbreak, as that will result in ultrasn0w being locked out.
Once you've jailbroken your iPhone using Pwnage Tool, fire up Cydia and search for ultrasn0w to get your carrier unlock installed, then just reboot your phone. Voila! Carrier unlocked.
More Info: iPhone dev team