Q: T-Mobile and 3G Nexus 7

I believe the 3G N7 will only work with AT&T.

Unfortunately, Google put some restrictions in place on the N7 that prevents you from using T-Mobile (prepaid or otherwise).

However, if you are willing to root your device you can use a neat little app called WiFi Tethering to use a different network.

To clarify, I believe the Nexus 7 is unlocked but comes with an AT&T card.

Regardless, the tablet can distinguish between a data card and a phone card. If you wish to go with T-Mobile be sure to get a micro-SIM card (not full size).

As long as the device is "unlocked" it shouldn't matter if it's T-mobile or AT&T, just make sure you use a "micro-SIM and NOT a full sized regular SIM card. The stores should have a device that will cut a regular SIM into a micro SIM