Ok I finally tried it – swapping in a T-Mobile SIM into an Apple iPad Air that was running a AT&T SIM for cellular data – and you know what, it works!
It was as easy as ording a Bring your own tablet T-Mobile Nano SIM at – http://prepaid-phones.t-mobile.com/sim-card?cm_sp=tmo_landingpage-_-yorktown-_-prepaidSIM
I got the card in a few days, popped out the AT&T SIM and insterted the T-Mobile SIM and set the SIM up for the iPad.
Here are some quick screen shots after I swapped the nano sim. I used a paper clip to pop the old one you, worked well.
First – screen shot of my iPad with the ATT Cellular Data