Yes, please someone try this. This has been a lingering question since the iPad was first announced. If I were American I would do it in an instant, partly for the curiosity, partly for all those gathered here who wonder.
I wouldn't use my iPhone SIM though. I would buy a T-mobile SIM with a data plan for a laptop, cut it down and test it. That way we can test if the iPads are truly unlocked (using T-mobile's compatible EDGE network) and if a miniSIM works in an iPad. Then I would call T-mobile and cancel the plan before there is any charge. It would cost 9.99 for the sim and answer a lot of questions.