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
And after I swapped it with the T-Mobile SIM
NOTE: You must DISABLE your Wi-Fi first so that you can get to the setup screen!
After that, you just have to walk through the menus and select the No Additional Data… option
Then setup your Cellular Data Info – never did it ask me for a SS or Credit Card number – NICE!
After a few more screens with agreements and information confirmation, you will get this screen
It does take a few minutes for it to all process on the T-Mobile side but once it has you will get a pop-up on your screen saying that it has been successfully activated. After that, you are all set!
Disclaimer: I accept no liability for the content of this post, or for the consequences of any actions taken on the basis of the information provided. I am just sharing my experiences and results, your experiences and results may differ.