Only available via the diagnostics menu:
Dial ##634#. This will open the diagnostics screen.
Dial *#7284#. This will open the tethering configuration dialog.
Select the option “Modem, tethered call”. The phone will take you through a set of steps to configure tethering and will restart the phone. Voila! tethering is now enabled.
Danger: You run the risk of bricking your phone, getting crazy bills from your carrier, etc. So do the above at your own risk.