[UPDATED] Bad news for downgrading 3.1.3 to 3.1.2....

Ahh....as David has pointed out, Apple is now verifying all firmware files before it performs a software restore. The only way to bypass this is to have done some pre-hacking on Cydia when your device was still jailbroken. I've looked around the net, and even after you have done this, it is still a pretty tedious process, so the bad news I bring is that....you will just have to wait for a 3.1.3 jailbreak!