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It’s a word of warning so take it seriously. If you have upgraded to the latest iPhone/iPod touch firmware 3.1.3 and now could not jailbreak for some reason and want to restore your older firmware 3.1.2 then let it be clear that Apple has stopped signing all older firmware and the only firmware it signs is the latest 3.1.3

Download iOS/iPhone firmware for iPhone (2G, 3G, 3Gs and 4), iPod Touch (2G and 3G) and iPad

This is what it implies:

  • Having upgraded to the latest 3.1.3 firmware you CANNOT restore to the older firmware 3.1.2

This also implies for the unlocked and un-jailbroken iPhones and iPod touch. If you have already jailbroken your latest firmware 3.1.3 using any of the available tools (Right now only redsn0w 0.9.4 has upgraded to support jailbreaking firmware 3.1.3) then you don’t need to worry about this.

To work around this issue of restoring to the older firmware 3.1.2 you might want to try using OpenSSH.

Update:

You can try using this guide on how to restore back to old firmware 3.1.2 (Direct download links for iPhone 2G, 3G and 3Gs firmware 3.1.2 are provided)