I had my contract renewed back in Jan and it din't include any tethering option. So I guess I am stuck for the next 1 1/2 years. However its great to see the new bundling option being offered. It really comes in handy when you are on the move or on vacation.
When I inquired the rep about how accounts that would still be under their contract plans -- what they told me was you'll still have locked to contract for full-term of the contract, however, adjusting rate plan / service (probably with some limitation -- like reducing lines) won't affect your remaining contract -- so you may want to check to see if there's any way you can make adjustments without any consequences.