[GadgetKen]

Another alternative to T-Mobile is the T-Mobile MVNO called Tuyo. Bought my card from an ebay dealer.
Pros: Cheaper IDT international long distance than direct T-Mobile, can use balance as calling card from regular phone when in non-coverage area, only $10 needed every 3 months to keep active, flat rate plan of 10 cents/minute with 10 cent connect charge, and a connecta2 access number for people to reach you toll free from Mexico and Latin America (I don't use all these features). Also has both English and Spanish prompts and customer service.
Cons: Only works on native 1900 Mhz T-Mobile network(fine for my watch phone since it doesn't cover 800 Mhz anyway); won't roam on other networks in US, Canada and Mexico like a regular T-Mo card can. Weak web browsing capabilities limited to their small Tuyo webstore. Also Tuyo refill cards less common so best to refill over the phone or web if not near a dealer.