ah hello! surprised you even understood my last post, horrid spelling.
Anyways though, you should be on att carrier 5.5 or 5.6 i belive before you try this, and even then i think the spell thing on here shall block repos, but we will see.
so this works for 3G and 3G[S]. get the latest carrier setting via itunes.
in cydia, add this repo from my 'awesome repos to have list'. REPO:
SiNfuL iPhone Repo
then after its done, search this exact phrase "enable tetgering for". you will see one for 3G, one for 3GS. choose your device, then REBOOT.
After reboot, go to settings > general > reset > and reset your network settings.
after it is done, you can tether. Voicemail, MMS and tethering all work on 3.1.2 just fine, and thata how.
NOTE: if you want toshare this, link it bqck to here and give credit =p