Unless 99.9% of T-Mobile users are unaware of this service

Or, worse yet, the users who are paying $20/month for their flat-rate bandwidth discover they can get it for free. T-Mobile may not notice that they're giving people free service, but I'll bet they do notice when people downgrade their calling plans. Maybe, just maybe, they're using this as a teaser to get people signed up, letting them turn the screws later. Of course, if other vendors find themselves forced to compete (offering high-quality undocumented services?!), then we'll have an interesting sort of competition.