[GadgetKen]

For Mexico it may be cheaper to buy a TelCel Amigo gsm prepaid card and keep it live via web refill (also can be refilled from the US website http://www.prepaidwireless.com/produ.../telcel_en.cfm ). But their rates to the US are fairly high, so best paired with a cheap Mexican prepaid calling card or callback service. You could probably do the same with competitor Movistar.
Other solution is some prepaid US phones will work in Mexico (Boost Mobile/Nextel iDen phones in Baja California, also my father's Verizon prepaid CDMA phone worked quite well in Cancun even though it probably wasn't supposed to...).
Cingular USA postpaid gsm also seems to have a good roaming rate of US$.99/minute for regular plans and US$.59/minute for their World Traveler option.