Go to Europe and splurge on a new iPhone there - they're sold mostly unlocked (you have to pay additional for that) and any SIM will work.

I haven't read up on this specifically since I'm not in the market for one, but I assume the 3G edition will be available for a high cost (plus awful dollar exchange rate) from those same European carriers in an unlocked format. Specifically, France's Orange provider sells them unlocked. There was a brouhaha about it last year, as there's some French law requiring phones be available without a SIM-lock as an option. I believe Germany followed suit. This year, with the much larger number of countries and providers, there may be additional options.