T-Mobile to Go now offers International Roaming, but I am not sure what countries are included. I know Canada is, and Mexico is likely as well.
"Can I roam internationally with a Pay As You Go plan?"
"Calls placed while roaming internationally, within other countries, will be charged a roaming rate + long distance charge. Please visit the Roam Worldwide section for information regarding current rates."
You will MOST likely need a GSM service to roam internationally. T-Mobile is GSM, and their pre-paid service now offers Intl Roaming. Very expensive though. You would be better served with a postpaid account with WorldClass Service activated, or buying a GSM SIM in the country you are visiting. Some might have extremely high rates back to the US (a prepaid intl SIM), but you can counteract this by using an international LD service like Onesuite.com
http://onesuite.com/access_international.asp
That way you could be charged "local" rates on your Prepaid Mexican or Intl SIM, and yet use Intl LD to call back to the US or other countries.