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.
Later edit:
Sn0wbreeze v1.4 is available. Fixes 3GS issues (Spinner) and adds iPod Touch 1G support. You can download it here