According to GeoHot's blog, "To fully enable tethering, download a MobileConfig file for your carrier. Watch out for carrier fees for tethering."
So, if you want to enable tethering, here are the steps to take (you will notice that steps below are the same as in our prior guide 3.0 and 3.1, but this time jailbreak is required:
1. You will need to jailbreak your phone. For detailed instructions, click here:
2. Unlock using BlackSn0w
3. Go to http://tr.im/oS1h
On your iPhone, in the Safari browser, go to the following website: http://tr.im/oS1h
4. Go to Mobileconfigs and download
Press Mobileconfigs. Find your country, select your provider, and press download.
5. Press Install and Confirm
The message reads: "IMPORTANT: The authenticity of "US AT&T" cannot be verified. Installing this profile will change settings on your iPhone." Changing settings is exactly what we're trying to do. Confirm with "Install Now".
5. Go to Settings>>General>>Network>>Internet Tethering
7. Bluetooth or USB
Decide how you'd like to tether. Follow the instructions on the screen (it's almost automatic).
* AT&T could be charging you. Nobody knows.
* Do this at your own risk.