How to: Downgrade iPhone 3G FW from 4.0 to 3.1.3 or 3.1.2

If you are a member of iPhone Developer Program you can go head and update your iPhone to the Next Generation Version which is iPhone OS 4.0. If you have tried OS 4 and want to get back to 3.1.3 or 3.1.2 you can follow this guide, Remember that the below step by step guide will only downgrade your FW from iPhone 4.0 to iPhone 3.1.3 or 3.1.2 or lower. It won’t downgrade your base-band. We will be using fuzzy band to downgrade your based-band.

Note: Make it on your own risk, we are not responsible for any damage for your Device, but I tried it on my device and it's working.

Steps:
Step 1: Download the firmware you want to downgrade then Download rslite.
Step 2: Now connect your iPhone to computer.
Step 3: Put your iPhone into DFU mode by using the step by step guide from Here.
Step 4: After putting your iPhone into DFU mode, iTunes will prompt a message “iTunes has detected an iPhone in recovery mode”. Click “OK”
Step 5: Then Hold down the Option Key and click Restore on your iTunes.
Step 6: Let the iTunes Restore, at the end of restore you will get error 1015.
Step 7: Now run rslite which you downloaded (it will open in terminal) and this will by pass error 1015. But if you got this message then go back to Step 3 and put your iPhone into DFU mode

Connecting…
Apple Mobile Device not found in Recovery or DFU
logout

Step 8: Execute the following

setenv auto-boot true
saveenv
reboot

That's it, you should be now with iPhone running 3.1.3 or 3.1.2.
If you had any errors, feel free to comment asking what you want and I will answer you.