Here is the guide to help you along the process:
How to Jailbreak 4.3 iOS Beta 1 with PwnageTool
WARNING: This is guide uses unofficial version of PwnageTool that may not be stable and will NOT preserve your baseband. So, if you rely on carrier unlock with ultrasn0w, stay away from it. Continue at your own risk!
Before you Begin:
Download iOS 4.3 Beta 1
Download Unofficial PwnageTool [Link Removed .. see Update at the end of the post]
Step 1: Launch downloaded PwnageTool in “Expert Mode” and select your device.
Step 2: Browse for iOS 4.3 Beta 1 IPSW
Step 3: Click on “Build” to create custom iOS 4.3 IPSW file
Step 4: PwnageTool will process the firmware and will cook custom IPSW that you can use
Step 5: Once custom IPSW is created, put your device in DFU mode:
Press & hold Power button for 3 seconds
Continue holding Power button + Press & hold Home button for 10 seconds
Release Power button but continue holding Home button until iTunes detects iPhone in DFU mode.
Step 6: Once in DFU mode, you can load custom IPSW into your device. To do this:
On Windows: Press & hold Shift button and Click on Restore button in iTunes
On Mac OSX: Press & hold Alt button and Click on Restore button in iTunes
Step 7: Browse for custom IPSW that you created with PwnageTool
That’s all you got to do to jailbreak iOS 4.3.