Can anybody recommend an international prepaid sim that works for GPRS in Mexico (on either network)?

None of the sims I have roamed (for GPRS) there, last time I tried. For example, Virgin Mobile UK (direct debit) didn't.

Plus points would be a cheap kB rate, and coverage in the US as well (for testing).

Negative points would be short expiry times, such as the Mexico domestic sims, or high start up costs.