International Roaming on T-Mobile
I had a Cingular/AT&T prepaid for nearly 3 years and it served me well for the timesI was in the US. Once I started travelling to Canada I found it didn't work. So I checked the other providers and found that T-mobile prepaid works internationally. I think you do have to activate their "Worldclass" service (no cost) before you leave the US. Their website has the details.
Hope this helps,
Glenn