iPhone Firmware 3.1 is great, unless you want to tether, which the tethering page at Apple will tell ya’ is not supported in the US (it’s not fine print but it’s smallish). The instructions at http://help.benm.at/help.php can get you able to tether in Firmware 3.0 (where it’s also not officially supported) and the video link on the site is also able to downgrade the firmware from 3.1 to 3.0 as you can see here:
http://www.benm.at/tutorials/howto-iphone-3g-firmware-3-1-downgrade-os-x