I can confirm that roaming data DOES NOT WORK on T-Mobile Prepaid.
The customer service rep was kinda confused by this (as am I). I know some people think that they make less money off of the prepaids, and maybe they do, but the logistics of running two separate groups would incur additional costs, not to mention that they already have my money. The postpaids they have to worry about people not paying. Plus, they have to operate the service for the month while the postpaids make 'interest' on that money. With prepaids, they get a fat bank accounts worth before the services are rendered. Seems stupid. Unfortunately, I roam too often to not get data so I'm dropping my service with them.