First, there have been some Data plan changes with some new T-Mobile plans. There is no total monthy limit on any of the plans and no charge for going over your plan. For example with the $10.00 / 200M plan, you can go over the 200M limit at no charge, but your connection will slow to the Edge netword speed after 200M. Going to a 2G or 5G plan gives you 3G or 4G speed up to your new plan limit before it slows down. I use the 200M plan because I am on Edge 80% of time anyway, so the slowdown after 200M doesn't matter.
Wifi tethering:
If T-Mobile detects that you are using your phone as a Wifi hotspot, they will email you saying you have to add the 14.95 monthly fee for tethering. There are many theories on how they detect tethering but it is obvious if you are surfing with Internet Explorer, they can tell you aren't surfing from your phone. (the browser and operating system can be logged by the proxy servers t-mobile runs on their data service)
It seems they cannot detect tethering if you only use it for Terminal Services (remote desktop). I have a workstation at home that I can access with Remote Desktop, so when Tethering with my Nexus S, I always go directly to my workstation at home with remote desktop. If you are not trying to display movies or other dynamic content, the bandwidth used is much less with Remote Desktop than if you were surfing directly from your tethered computer, and T-Mobile cannot tell if the Remote Desktop app is on your phone or on your tethered computer.
This solution is good for email and accessing other non-animated apps and web sites. Not usable for anything that moves or tries to display a lot of big images.