You implied that there is more competition between AT&T and T-Mobile because of GSM. There's only one phone that can go between both, so that theory is wrong.
That, and AT&T and T-Mobile don't compete in the same market space anyways. AT&T and Verizon do.
I still don't understand what you are taking from my original argument but I'll try to summarize it again. I was trying to respond to another poster who was asking why prepaid doesn't really exist in the US like it does elsewhere.
The prepaid market relies on the ready availability of phones that can easily work between carriers. Prepaid markets have high turnovers with customers usually switching from one to the other based on prices.
In the US, where the only two GSM carriers are on two incompatible 3G GSM technologies makes prepaid almost non-existent here. GSM for prepaid matters because the mass majority of phones produced overseas are GSM. Not to mention its easier to switch carriers - you just need to throw in a SIM and go.
Add that only other large wireless provider, Verizon, does not use GSM (thus requiring you to purchase a Verizon device to use their prepaid service), and the fact that AT&T essentially has a stranglehold on the GSM market in the US, translates to a wireless market in the US where prepaid does not exist.
T-Mobile does put some downward pressure on AT&T - but the fact that their 3G is not compatible with a majority of the AT&T phones really hurts how effective that is.
If T-Mobile's gamble pays off and they can refarm and enable 3G on 1900 (thus allowing unlocked AT&T phones to work on T-Mobile) - you can bet competition will heat up between the two.
You can also bet that if both Verizon and AT&T used GSM - prices in the US would be a lot lower.
