Update; As of this Friday, T-Mobile now has a roaming agreement with "Telecom Personal/PERSONAL/AR TP". A device supporting GSM1900 is required. Coverage is select cities only (Buenos Aires, Cordoba, Rosario, etc)
http://www.t-mobile.com/internationa...geInternet.asp
GSM World map:
http://www.gsmworld.com/cgi/ni_map.pl5?cc=ar&net=tp