ATTENTION! If you have 3.1.2 running on your iPhone/iPod Touch, DO NOT upgrade to 3.1.3 !! you may get a message when you plug it into iTunes but do not install it!! The only thing it does is improve battery life readings on iPhone 3GS.
If you think that's worth losing your jailbreak for, go right ahead!
And remember, as with any new apple firmware update, it will clear out all jailbreaks and you will have to wait for developers to release a new one. Stay posted for a revised 3.1.3 jailbreak tutorial!