Originally Posted by themax
So far it doesn't seem like the Hotspot mode requires the specific plan. I tested it out on my phone. The only message I got before enabling Hotspot mode was that the feature required the service on my account, afterwhich it enabled anyway (no message for any agreement for additional service) and I used it just fine.
Now we just need to wait and see what your next bill looks like.
Since it is a carrier-specific phone, even with a mostly-vanilla Android build on it, I wouldn't be too surprised if the tethering app had been modified to notify T-Mobile that you were using it.