Issue:  My iPhone 3GS was working fine prior to upgrading to iOS 4.  Once I did that, I couldn’t connect to the 3G network anymore even though all the apps worked while connecting via wifi.  I tried turning the Airplane Mode on and off, rebooting, etc. etc.  Nothing worked.  It was completely frustrating!

Fix:  YMMV, as I’m on the Fido network in Canada, but this what I did.  I have to do this every time there is an update as well.  I went to Settings/General/Reset/Reset Network Settings  Once I did this, the iPhone restarted, searched for a network, and everything worked again.

If you try this, you’re on your own, but it worked for me.  Good luck!

Well, as of iPhoneOS 3.1, the iPhone will not accept unsigned config files (unless you have jailbroken); as such, this file will no longer work. This means that if you have the unsigned config file, your tethering option will disappear.

However, seeing as Rogers/Fido has officially embraced tethering (for now). If you have the hacked and unsigned config file installed it (or any similar file for other carriers, like AT&T, T-Mobile, etc.) and upgraded to iPhoneOS 3.1 or 3.1.2, you will need to remove it.

Open Settings > General > Profiles (at the bottom). Remove the profile you see there. Tethering should now show up for you.

Enjoy! The credit for this goes to http://cdslash.net/about/ who helped me work out the whole config file thing in the first place…