The bad news is that you may lose your tethering option after updating to iOS 4 from your previous iPhone OS 3.1.3 installation. The good news is that my 3.1.3 solution still works with iOS 4, so just follow https://www.kaisblog.de/2010/02/19/reenable-iphone-tethering-313/ to re-enable tethering on iOS 4. Good luck.
Re-enable iPhone tethering with iOS 4
Jun 8th, 2010 by Kai
Tried it and it doesn’t work. Or rather I can install the profile over wi-fi after restoring to factory settings, but after restoring my settings from the backup there’s no Internet Tethering tab. Try reinstalling the profile and it says that there can only be one profile installed, but there are no active profiles listed.
Update to the last comment – restored to factory settings and got the phone to take the profile, but no tethering option reappears in the settings > general > network section.
Sounds like your iPhone is not factory unlocked.
same here, tehtering ist there until i restore my backup profile, the tethering option was gone. factory unlocked, e-plus.
Never experienced this. Maybe we need to remove the old tethering profile before making a backup.
I removed the profile, backed up, restored with manually entered apn, tethering disappeared after restoring the backup. 🙁