Few days back I had given links for Free download of iPhone 3.0 beta firmware. Many of the people who downloaded were happy with the upgrade but few of them faced issues with it and wanted to downgrade to 2.2.1 firmware version. Below are the steps to do the same (via Crenk)

How to downgrade iPhone 3G 3.0 back to 2.2.1?

Firstly, connect your iPhone to your Mac or PC and select the iPhone from the list of devices in iTunes. If iTunes does not detect your iPhone then you will need to put it in DFU (Device Firmware Upgrade) mode. You can follow these steps to put your iPhone in DFU mode (this is required only if iTunes does not recognize your iPhone):

* Connect your iPhone to your computer.
* Turn iPhone off.
* Hold power and home together for 10 seconds (exactly).
* Release power but keep holding home until the computers beeps (observed on a PC) as a USB device is recognized.
* A few seconds later iTunes should detect your iPhone.

Browse the Summary Tab. For Mac, hold down “option” and press the “Restore” button. For Windows, hold down Shift and press the Restore button. Now you will be able to browse the folder where you have the iPhone firmware. Select the firmware and hit the restore button and cross your fingers for 5 minutes!!

If you get error message like 1011, 1013, 1015, just ignore them(Actually the firmware 2.2 is already successfully installed). Fire Quickpwn 2.2.1 and jailbreak your iPhone use this guide. But if you encountered with error message like 1601, 1602, 1600 or anything else then try putting the iPhone into DFU mode and try again.

If you have an Unlocked iPhone then use pwnage tool to make a custom firmware then enter DFU and restore / downgrade to 2.2.1 if not works in first time done it again n again.

Disclaimer: This tutorial is for informational purposes only. Do it at your own risk.