Proceed with this information at your own risk. We are not responsible for any damage to your phone. Read directions completely before you proceed.

1. Connect your iPhone to your computer. Allow iTunes to launch on its own. If iTunes does not start on its own, open it manually by clicking on the desktop icon on your computer.
    
2. What is the status of your phone?
   
   
   1. Working: Power down your phone as usual. Hold the power button until it slide to power off. Wait for the phone to completely turn off and the screen to go black. After 10 seconds, goto step 3.
       
   2. Not working: Hold down the "Power" and the "Home," the round button with the square on it, buttons at the same time until the phone turns off. This should take about 20 seconds. Wait until the screen goes completely dark.
       
3. Hold both the "Home" and "Power" buttons, at the same time. If the phone doesn't turn back on, release both buttons and repress them. Within about 10 seconds, the apple should appear on the screen.
   
   
    
4. Release the "Power" button, but continue to hold the "Home" button. Once the iTunes icon appears on the phone, release the "Home" button. iTines should recognize your phone and it should show up in the devices list. You should see this on your iPhone:
   
   
    
5. Hold down the "Shift" and "Alt" buttons on your keyboard at the same time and press the "Restore" button in iTunes. Open the original firmware file by clicking on it, instructing iTunes to downgrade to this version.
    
6. Wait for the downgrade process to finish. You may receive an error message due to the fact that you have not allowed iTunes to complete the upgrade process as planned, but you can ignore it by pressing "OK." You should see a message on the iPhone's screen though, reading "Connect to iTunes" with a small yellow triangle next to it, which means the downgrade has been successful.
    
7. Turn off your iPhone and restart it before using it. This will confirm that your iPhone is working properly and will also notify AT&T to restore your original service to the phone.
   
    

Great iDevice support site: http://forums.mactalk.com.au