First thing you will need to do is SEARCH! SEARCH! SEARCH!
the videos and tutorials ARE out there.
I made the foolish mistake and upgraded to 3.1 a little google searching i found this.
3G USERS CLICK HERE
Revert Back From iPhone Firmware 3.1 To Firmware 3.0.1 | Jaxov
3GS USERS CLICK HERE
that worked flawlessly taking me back to 3.0
then i went ahead and istalled
for my carrier which is ATT this firmware gave me visual voice mail and mms as well as my tethering option. if your carrier is different you will have to find your old firmware or comparable firmware
again do a search on google its full of info.
too lazy? click here.
HOW TO: Install Official MMS on iPhone 3G/3GS (3.0-3.0.1) (Jailbroken or Not) - Mac Forums
i got my carrier file from there and it worked perfectly.
Finally I came here. Good old hackintosh forums and used the ultrasnow tutorial and re j/b my phone.
AND ALL OF THIS TOOK LESS THAN 30 MINS! With my original searching.