If you have the T-Mobile grandfathered international rates you are golden. I was a T-Mobile customer since 1999 as well. Just recently I though of switching to Cingular (they use GSM in certain areas). But I decided to stick with T-Mobile because of service prices.

In terms of future technology I think GSM combined with 3G (like they have in Europe is the way to go) We should see something coming out in the US within a year or so.

Cingular is way more expensive than T-Mobile, especially when it comes to additional services (email, Internet, international roaming, SMS,etc) The poor quality of reception is not the property soley of GSM but also of carriers which focus on selling wholesale minutes rather then QoS.

Verizon and Sprint, in my opinion, are junk. Verizon is ok with high speed internet.

With GSM you can get any international phone you like, forgo the obsolete models offered by T-Mob and Cingular. Make sure you get a good phone (like Nokia) for good reception and sound quality. The rest is up to network coverage really.