There's an unofficial version of PwnageTool that's already available for the iOS 4.3 Beta. It only works on the iPhone 4 at this point, and keep in mind that it's unofficial (with custom bundles) it won't preserve your old baseband. Unlockers should stay away. Please use this jailbreak at your own risk. In fact, we'd stay away from it altogether, but since we are in the business of reporting the iPhone happenings, it's also our duty to post this information. On the bright side, 4.3 is only available to users with a developer license. If you don't have one, that's $99.
Important: Do not forget to backup your device if you choose to try out this jailbreak.
Here's what you need:
Current version of iTunes: Here
iOS 4.3 Beta: Only Open to Developers
Once you have all that, the process is exactly like the iPhone 4.2 jailbreak: Here