The non-carrier locked iPad 3G won't run on other networks without a little love. That love was detailed today by iPad Insider in this untested DIY project.
Yes, untested. iPad Insider stumbled across the DIY project and accompanying video, but has yet to test it. We haven't tested it out either. Proceed at your own risk. Also, lower those expectations, this is Edge.
1) Have your iPad 3G and T-Mobile SIM with EDGE data plan ready
2) With the SIM eject tool that came in the box, take out the AT&T Micro SIM and place it on top of the T-Mobile traditional SIM, trace it and trim it with a scissor or any other tool that can get the job done right (Be sure not cut the gold contact). Or follow this guide from Techradar.
3) Insert the DIY T-Mobile SIM in the iPad
4) Turn off WiFi
5) On the iPad, go to Settings > Cellular Data > Turn Cellular Data ON
6) Turn ON Airplane Mode and then OFF to refresh
7) You should be on T-Mobile EDGE at this point, if not go to Settings > Cellular Data > APN Settings > And try these on the "APN" field:
Saving money with this set up is not guaranteed, but if you happen to have one of those grandfathered-in $5.99 T-Zones data plans (since discontinued) it could be worth checking out. Again, did I mention this is completely untested beyond what's shown in the video and these instructions?
The video walk through is below:
Downsides: Slow. You lose the use of your phone while this DIY project is in effect. [iPadInsider]