For the IPhone to work in the USA, there are a few networks that they could use. AT&T & TMobile run GSM (though on different bands). TMobile's network is tiny compared to AT&T. However, GSM is pretty well understood worldwide and is used in most European countries. One design, one radio, one spec to write code for.
Verizon runs CDMA, as do some Asian countries. In order to support Verizon, it requires different hardware and new API to access CDMA.
Then there's Sprint which runs EVDO, requiring a separate radio and API.
So, in the USA, we have 4 major networks running 3 different standards. And TMobile/AT&T don't even use the same GSM frequency.
In Europe, they have a fairly unified, universal standard which makes it easy to have one model of the IPhone that works on multiple carriers.
While I know a lot of people have had problems with the AT&T network, in South Florida, I have found it to be more reliable than Verizon and Sprint. I've not used TMobile, but, have talked with people that have and while call coverage is good, they routinely complain about SMS delays.
AT&T did approach Verizon and Verizon didn't want the IPhone with Apple's terms and conditions. With CDMA, there is no talk and surf, so, an early complaint of the IPhone would be a technical/network problem with Verizon.
Personally, if a droid or droid-like phone were on AT&T, I wouldn't hesitate.
HTC Lancaster was announced for AT&T, then held back until 1Q10 and recently seems to be shelved.
Apple did turn down
iDroid 