I have the same plan. Tethering is an additional $15/month, for a total of $45. Still a far cry from $90. I use it in Portland and the service is great. (Total after taxes is $48.50)
The problem I have with T-Mobile is their customer service reps are so poorly trained.
So depending on who I speak with, I am told I either need to pay the $15 for tethering or not. I pay it as I am a bit afraid to drop it never to have it return.
So can you tell me more about how you much you tether, and how often? Or if T-Mobile has ever given you anything official about your tethering being okay?
When I moved into my new apartment I used it more than usual (maybe 2-3 GB over a few days) and they started redirecting my traffic to a "Pay for Tethering" site. It's just user-agent based though so very easy to circumvent. Other than than I have had no issues.