After posting my original question, I've done some more research into this, and thought I would share what I've learned. GSM is the best for Europe, Asia, and major metropolitan areas in the USA. Here in Nevada, Cingular provides GSM, but almost everywhere else, it's Voice Stream. AT&T has just added GSM service in the states. But, like Detroit Diver said, TDMA is the best for the Caribbean. (With the exception of the French islands; they use GSM) Mexico and points south, look like they use CDMA, as well as analog. Some Caribbean islands, such as Grand Cayman, use analog, too. Nextel confuses me, so I won't even try to guess about them. GSM seems like the wave of the future, but wouldn't help me in the Carib. I read somewhere you can buy a prepaid phone card on Grand Cayman for use with your TDMA phone, but I'm not sure if you can receive incoming calls. This is all just info I've read, and is no substitute for real world experience, so thanks everyone for responding with your personal experiences. Hopefully, we can get even more input, as we all continue to travel.
And, Tavi, don't you like to sit on a beach somewhere tropical and call your non-diving friends back home, just to gloat good-naturedly? Just a little?