Apple today released a new version of iPhone OS. This update contains 3GS baseband upgrade (05.12.01) so BE WARNED. DON’T UPDATE YOUR iPHONE 3GS.
There is not any notable change in this firmware.
iPhone 2G users : Can use redsn0w (download here) in order to jailbreak it. Please note that you should have firmware 3.1.2 file and you must point redsn0w to that files after updating (or restoring) to 3.1.3.
iPhone 3G users : If you need to unlock your iPhone baseband (using ultrasn0w, etc) DON’T UPDATE TO 3.1.3. otherwise use the above method.
iPhone 3GS users: BE WARNED. DON’T UPDATE YOUR iPHONE AT THIS TIME.
iPod Touch 1G users : Same as iPhone 2G.
iPod Touch 2G users : Do not update to 3.1.3
iPod Touch 3G users : Do not update to 3.1.3
The above information maintained from Dev-Team blog