Verizon offers a couple of "global" phones that have both CDMA (for use in the USA and South Korea) and GSM (for most of the rest of the world). No luck in Japan.

http://www.verizonwireless.com/b2c/m...hone/index.jsp

The phones are expensive, and the service abroad costs more than a dollar per minute, but it allows you to have Verizon's CDMA network in the US and still access GSM networks overseas all with a single handset and phone number. Unfortunately I don't think any of their CDMA/GSM global phones have AMPS (analog) capabilities, so you'd be giving up rural US access.

IMO, the forwarding service + unlocked GSM handset + local country SIM solution makes more sense. But then you'll have a gap in your connectedness while you hunt down a new SIM card in each country. And prepaid SIM cards in some countries are localized so they can't be easily "topped up" in cities other than where you bought them.