My PDA phone is an iPhone with ATT's international data plan. It is great for VOIP over 3g, triggering call backs, etc. My prepaid SIM goes into a Nokia E66 (2100mhz). It is a great phone for prepaids and works well with most networks. It also tethers easily.

have two apps on the Nokia that thought deserve a plug. The first is Roam2call which is a Swiss Army knife of tools. It can be programmed as a call through utility (dial a number, enter a pin, and continue to dial), a call back utility where the phone calls you back with a dial tone, and an SMS callback utility . I haven't used it, but a friend has gotten it working with one of the Betamax services.

If Argentina still has all that TDMA, I'm wondering about that old Cingular hybrid TDMA/GSM phone. I wonder if ATT (which absorbed Bell South which installed many of the South American networks) could get it roaming on those networks. It wouldn't be cheap, but might work.