As of right now, it's a block on the phone's side. There's a process called 'Tethering Manager' that runs on the newer phones (like the Galaxy S II) that block tethering if you don't pay for the add on. Removing/freezing the service means you can tether freely, so that's what I did via rooting.
I don't think you have anything to worry about on your older phones that you use to tether. That should continue to work until T-Mo finds a better way to weed out the people who don't have the tethering add on.
BTW, you should consider talking to the retention department. There's a "preferred" data plan (5GB then throttled) for $20/mo that most here on HowardForums have, including myself. You'd have to call 611 and say "loyalty department" at the prompt.
Castrol EDGE 5W-30 is my drug of choice.
Toyota | moving forward
Primary: Galaxy Note II & iPhone 5 (AWS): 5 lines, 1x $15 4.5GB data, 3x $2.99 T-Zones (truly unlimited 4G data), myFaves, corp discount + $10/mo loyalty discount = $88/mo tax inclusive
Secondary: iPhone 3GS: Airvoice Wireless, Samsung Fascinate & HTC Trophy: Page Plus