Actually, I won't upgrade to iPhone until Apple opens up their phone to Verizon. AT&T has horrible signals in my area. In addition, I still have a good amount of a 2-year contract w/ Verizon. Why in the heck did Apple choose AT&T?!?!
Apple initially tried to deal with Verizon.
<http://www.usatoday.com/tech/news/2007-01-28-verizon-iphone_x.htm?POE=TECISVA>
And why won't they let other carriers support their phone?!?!
AT&T is the only US carrier that is compatible with current iPhones.
Verizon uses CDMA, so Apple would have to develop a special phone for them, but Verizon is in the process of going away from CDMA, so that development would be for a phone with a very limited market (one carrier, and for only a short time).
T-Mobile uses GSM, so the iPhone would work, but T-Mobile uses non-standard 3G frequencies, so speed and some functions would be limited. Some of the new Canadian carriers also use those 3G frequencies, so Apple could add the frequencies to the iPhone (that would be a lot easier than adding CDMA), but those carriers lave limited coverage, so again, the market would not be very large.
In a few years, when Verizon has finished its conversion, things may change.