Did you accidentally upgrade to 4.0? Or are you just now regretting the switch? Fear not, there is still a working method to downgrade, if you have a Mac, that is.
What You'll Need
How To Downgrade
Install the LibUSB package
Connect your iDevice by USB, launch iTunes, and Alt-Click on the restore button
Select the 3.1.3 firmware you previously downloaded. Wait for it to fail during the restore and give you an 1015 error. Now close iTunes.
Open Terminal. Drag the iRecovery file onto the Terminal window.
- [spacebar] -s
- setenv auto-boot true
Close the Terminal window, restart your iDevice by holding down the Sleep and Home buttons together until it reboots, and open iTunes to continue with the set up.
And, there you go, one iDevice perfectly downgraded to 3.1.3.