If so, which terminal? (Hopefully Terminal D.)
Specifically I'd like to buy a prepaid SIM card ideally without leaving IAH on a BYOD 'Simply Prepaid' T-Mobile plan for just one month while I am travelling around Southern USA and the Caribbean.
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.
Yes. There is a Best Buy automated kiosk in Terminal D that offers no-contract cellphones you can purchase. If you visit here and filter by location to Terminal D, you will find more information.