T-Mobile's implementation requires a driver to be included with the phone's firmware, as well as the app to adjust the settings. The app is available through the store (for supported devices), but the driver can only be obtained through the phone update settings screen, and obviously only when HTC includes it and T-Mobile chooses to release it. I think the problem is at T-Mobile or maybe HTC's end.
What I find really interesting is that it appears that T-Mobile's testing process doesn't even attempt to test out their own apps. If they had, they'd have noticed it was missing. I wonder what their tests do involve?