Downgrade iPhone iOS 4.0 to 3.1.3

Few days back Apple released iOS4 which is compatible with the iPhone 3G, iPhone 3G and iPod Touch iPod Touch 2G and 3G. Most users updated their iPhone to OS  intentionally or accidentally and now finding way to downgrade back to previous iPhone firmware. If for any reason, you upgraded it to iOS 4.0 and would go back to 3.1.3, follow the steps listed below.
To downgrade the firmware on the iPod Touch and iPhone 3G, you need the ECID SHSH certificate for firmware 3.1.3, and we are sure that you had saved it before the official release of firmware 4.0.

Steps to Downgrade iOS 4.0 to 3.1.3 OS Firmware

Step 1: First we make a manual change to the hosts file according to the Windows operating system. Navigate to the path “c:\windows\system32\drivers\etc\ and open the hosts file in a notepad or wordpad. MAC users can navigate to the path “/etc/” and open the hosts file with TextEdit.
Step 2: At the end of file add the string “74.208.10.249 gs.apple.com” useful to redirect requests for certificates for authenticity that iTunes will make to the Saurik server.
Step 3: Set up your device in recovery mode.

• Turn off and unplug your device from your computer
• Now press and hold the Home button while reconnecting the USB cable to your iPhone. After iPhone is connected it should power on the device.
• Continue holding the Home button, you will see the iTunes logo appear and then finally a “Connect to iTunes” will be displayed, now is the time to release the Home button.
• Once the logo appeared in iTunes, you can release the Home button and make a normal recovery

Step 4: Now open iTunes and do a normal reset by holding down the SHIFT key (for Windows users) or Alt key (for Mac users) and clicking restore in iTunes
Step 5: Now select the original 3.1.3 firmware and wait for the completion.
Voila! you downgraded from firmware iOS 4.0 to firmware 3.1.3 and you can follow the normal guides to jailbreak firmware 3.1.3