Not sure what you are exactly asking but here is how it worked for me...
I had the grandfathered unlimited data plan.
It was $30/month (we are talking AT&T of course).
I lost my unlimited and got the 4GB Tethering pro plan for $45/month.
The personal hotspot feature is in the Network settings in iOS4.3.
As soon as I switched plans on the web, the personal hotspot feature started working.
You will lose your unlimited forever if you do.
I never came close to even 1gig a month, I think I can keep it under 4.