There is also T-Mobile USA with GSM/3GSM that only requires a simple spectrum added to already used and known chipsets.. They may be in 4th place but they still have about 35M subs, or 40% of AT&Ts total. They may even build their subscriber base by 10-20% (3.5M-7M) in the first full quarter, thus closing that gap quite a bit.
Also, Apples been working with the parent company in many countries and T-Mobile USA seems very likely strike any deal that Apple wants unlike the current and well known leader in the US, Verizon.
Yes, T-Mobile also(something i forgot to add.)
Also, Apple wanted speed in it's device, like the Safari browser, application downloads, etc.
That's a certain reason why it's still has a better chance on staying with the AT&T network.
But, obviously, not to be a douche, i'm not gonna criticize T-Mobiles network because I currently don't know how fast their data works.
Who knows, Maybe Apple will strike a deal with T-Mobile in 2010 & both wireless companies can share the iPhone.