Do a search of “tethering the iphone 3.1 at&t” and you get 102,000 hits and the first 10 are worth nothing. Why? Because they only tell you that Apple did break the tethering over USB/bluetooth for the iPhone 3GS version 3.1.2… so what am I writing about then? I am writing about “Enable Tethering for the Iphone 3GS” from SiNful iPhone Repo, one of lots of applications in Cydia (yes my iphone is jailbroken, but it is my hardware).
Now, my iPhone gives me lots of download speed. I was controlling my home server over logmein, over the iPhone.
So, forget Google and choose Cydia.