Hmm. I think SIM cards are part of the GSM phone standard - but I could be wrong. However, some phones are locked to a particular service provider (e.g. when you buy a "reduced price" phone from Cingular, and they have put a lock on so that the phone will not accept a SIM from another service provider). CDMA may not be using SIM-cards, I don't know much about CDMA. [I used to work on GSM base-stations, so I do know a bit about that]
It's worth looking into which service provider covers the particular area tho', since although other service providers will allow "roaming" (that is, you can use other networks), you will be paying extra for that service. Or it may be "blocked" from your service. Read the fine-print on your subscription contract.
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Mats
