In the USA there are 4 nation wide phone carriers. Two of the four use GSM based phones. The other two are CDMA based. I like to be able to move from phone to phone often so it only makes sense for me to stay with a GSM carrier. But it looks that the liberty I had to move from phone to phone may be slightly eroded. The larger of the GSM carriers (AT&T) is no automatically adjusting subscription plans when the SIMs are moved into certain phones (heard via The Cellphone Junkie show 194). I am currently with T-mobile and found out first hand that they are now enforcing some level of policies on their SIMs too. Some one asked me for a GPS navigator so I put my data only SIM in my T-mobile myTouch 3G so that the person could use Google Maps with Navigation. A few hours later the phone could get no data with the exception of the browser. Any request made on the browser resulted in a page being returned allowing one to purchase a voice plan; T-mobile doesn't allow data to be used in that phone without a voice plan.
These restrictions are pretty light. But if the tendency continues (and if the policies become more restrictive) then the advantages of using GSM start to diminish. If they diminish enough then the GSM phones will have the same type of inflexibility that the CDMA phones have. If that happens I may have to take the other two carriers into consideration.