While yes there is indeed a Best Buy vending kiosk in terminal D (and several other terminals) at Houston IAH, it does not (at this time) stock T Mobile SIM cards or phones.
There was a variety of electronic items such as MP3 players and headphones, and a 'SIM International' SIM starter pack. No USA based carrier SIM only kits were offered in this machine.
After speaking with Best Buy staff I was told that they no longer stock T-Mobile SIM cards at all, only several other brands.