The problem you will run into is the different technologies available in phones currently

There are almost no phones currently sold in the US that can do both CDMA and GSM systems in europe they are quite common (I had a quad band V60 I bought in germany when they first came out great phone and worked everywhere too bad it got stolen) I know there is one or two from Verizon (one samsung for sure and I think a nokia) but from other carriers I see a lot of GSM only or CDMA only phone programs which still leaves a huge gap in the easy rollover of phones from carrier to carrier...I just wish Verizon would allow you to use the full capability of the bluetooth system in the motorola phones like alltel does.