With the new Firmware 3.1.3, Apple didn’t just bring bug fixes and hidden bug fixes but they also focused on breaking the jailbreak. And so far , they were successful , because the new OS seems to be partially hopeless. This is important for all iPhone 3Gs, iPod Touch 3G and iPod Touch 2G model MC users.
As you might now, people that own an iDevice with the new iBoot, before they upgrade or restore a firmware via iTunes, they are sending a direct request to Apple’s server. The server is receiving the request, and if it approves it, the restoration is successful.
But today Apple has removed the digital signature for the firmware 3.1.2 and all the iPhone 3Gs , iPod Touch 3G and iPod Touch 2G model MC users users will not be able to downgrade once they upgraded to 3.1.3, unless they saved their ECID certificate through Cydia or Rock.
If you are a jailbreak user running a iPhone 3Gs or iPod Touch 3G and iPod Touch 2G model MC that has not stored the TSS in Cydia, you should be very careful with what you do from now until the next jailbroken firmware. You will want to be careful in what you install to make sure you don’t have to restore for any reason. It would be a good idea to install openSSH so that you can get in and fix any problems without a restore, also.
iPhone 2G/3G users need not worry as you dont need Apple to sign your firmware files.