TMO representative told me yesterday "We're working on it" regarding international roaming, and that it should already work on Telefonica in Mexico. However, "we're working on it" has been their standard answer for the past year or so, so I'm not holding my breath, even though the first overseas networks I would expect it to work on would be T-Mobile's own, and not one belonging to its Spanish arch-enemy.
The FAQ now seems to imply it is an option:
http://www.t-mobile.com/templates/faq.aspx...e#howtorecharge