So you want to know how to enable tethering on 3.1.2 well it’s not as hard as you think. You will need access to a version of OS X that has bspatch installed. I’m not sure which version as I’m not a mac fan.
You will need either of these:
You will need to install MobileTerminal (available in Cydia) and also OpenSSH (from Cydia) if you plan to copy the files via SSH. I used i-FunBox just to make it easier.
Open i-FunBox and navigate to:
Copy the file CommCenter to your desktop and transfer to the desktop on your MAC along with the CommCenter.patch file.
Open Terminal and run the following:
bspatch CommCenter CommCenter-hack CommCenter.patch
Now delete the CommCenter file and rename CommCenter-hack to CommCenter. Rename the CommCenter file on the iPhone to CommCenter.bak and copy over the new one.
On the iPhone open MobileTerminal and run:
su – (password is alpine)
chmod +x CommCenter
Then restart you iPhone. You should now see the Tethering option in Settings > General > Network. If not, visit http://help.benm.at/ on your iPhone and install the MobileConfig file for your network.
Note: I take no responsibility if this ruins your iPhone or you get charged for tethered data.