As you know, firware 3.1.3 is updating the baseband so, if you are depending on unlock, you should not update. You can downgrade from 3.1.3 to 3.1.2 ( need firmware 3.1.2 because Apple is not signing it anymore ) if you really want to ( won’t downgrade the baseband though ) , but don’t expect the DevTeam or GeoHot to release any tool updates soon.
If you really want to update to 3.1.3 and still be able to unlock, you still have a hope: Sn0wbreeze. Currently the tool is in private beta, and it supports all iPhones. Over the next two weeks , support for iPod Touch will be added ( we will see what models ). As soon as the tool is out of private beta, we will let you know.
Sn0wbreeze v1.4 is available. Fixes 3GS issues (Spinner) and adds iPod Touch 1G support. You can download it here