[ajg1977]

You're mistaken.

I never unlocked my 3GS but had tethering enabled for most of the 3.x lifecycle.

[sirn]

I am pretty sure installing unsigned MobileConfig was disabled in 3.1.2. Maybe you already installed carrier update that has tethering enabled?

[spicyj]

Well, in any case, installing the profile from

http://m.peacefulinsanity.com/Tether.mobileconfig

didn't enable tethering on my 4.