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:
Blacksn0w Unlock (05.11.07 Baseband) and Blackra1n Jailbreak for iPhone How-to Guide
Like Pie? Download PwnageTool to Jailbreak iPhone 3.1.2 : UltraSn0w Unlocks 3.1.2
2. Unlock using BlackSn0w
1…2…3… Blackra1n Unlock is Available Via Cydia
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).
WARNINGS:
* AT&T could be charging you. Nobody knows.
* Do this at your own risk.


















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While this method does allow for tethering. It seems to interfere with the iPhone 3Gs ability to pick up wifi.
Does push fix solve the wifi problem?
Grz, Steffel.
Since phone has to be jailbroken for this method, I would recommend trying PDAnet
http://www.ismashphone.com/2008/09/pdanet.htm (you can buy it in Cydia, it’s no longer free) but it’s worth it.l
I tried the website on safari but it keeps giving me the message the could not be found
No problem here. Try emptying cache, that can be a problem