I came across something interesting on T-Mobile's website.
http://explore.t-mobile.com/phone-sim-card
I went to get a sim for a relative's AT&T iPhone and "Step 1: Check Your Phone" lists "Apple Sprint iPhone 5" as a compatible device.
Also, the fine print says this...
"Only T-Mobile and AT&Ts iPhones, and Verizons & Sprints iPhone 5 will work on our network; other iPhones may have limited functionality, including coverage limitations. Capable device required for 4G speeds; capable iPhone may achieve 4G speeds in limited areas. iPhone is a registered trademark of Apple Inc."
Has anything recently changed?