Two options to consider. One is an toll free number in the states that will forward to your prepaid number in each country. This would require you to set the forwarding number whenever you switched SIMS but would be transparent to the people calling you. They would call the same toll free number. Second would bt the use of a Hopmobile or some other international prepay. Check the rates for the countries you are in and see if it isnt cheaper than roaming. Also, these are not mutually exclusive solutions. For instance you can get a US toll free number and forward your Tmobile to it while it is set to go to a prepaid. If you move to a country without a prepaid or cant switch the toll free forwarding number you could just cancel forwarding and your calls would come to your Tmobile phone. That would allow you to save a lot of money when you could forward the calls. Or perhaps even forward to the Hopmobile if you couldnt get a local or forward to it through the toll free number.
You could also use a call back system or calling card for calls you originate which would save some money. It would be worth it to get a second phone with a local sim for this.
Here are two threads describing using the Telcan system for the toll free number and call back in more detail. I use it and am happy with it although I think there may be cheaper rates from other companies.
http://www.howardforums.com/showthre...+%2Bforwarding
http://www.howardforums.com/showthre...n&pagenumber=2
Note that one of the threads starts talking about using it in Japan where a TMobile phone wont work and you would have to set up the forwarding before you leave. In GSM countries that isnt necessary as you can change forwarding on your TMobile phone anytime since the phone works there.