Apple has stopped signing iPhone OS 3.1.2. This means that those with an iPhone 3GS or iPod touch 3 who did not use Cydia to store their TSS cannot restore to anything but OS 3.1.3.
If you fall into that category, Big Boss warns to please make sure you are very careful with what you do from now until the next jailbroken firmware. Be sure that you don't do anything to your iPhone that may require a restore.
You may want to install OpenSSH and learn how to use it so certain repairs can be performed without a restore.
iPhone 3g and 2g users do not need to worry about this.