Quote:
Device must be jailbroken on 3.1.2 before upgrading using custom 3.1.3 firmware. Note: Apple no longer signs any firmware besides 3.1.3. Only signed firmware can be installed on an iPhone 3GS, which means once it has been upgraded to 3.1.3, it cannot be downgraded to 3.1.2 and hence, cannot be jailbroken. The only exception is if the device's SHSH file has been previously backed up while it was jailbroken. If an iPhone 3GS user upgrades to 3.1.3 without having their SHSH file backed up, it is impossible to jailbreak the device.
So I believe - it aint gonna happen.