I second everything that Tubmleweed mentioned since it's dead-on accurate. The real debate is between Verizon (great coverage) versus Sprint (great 3G speed), with AT&T and T-Mobile being runners up.
Be aware that if you plan to "tether" (connect your phone to your computer to let your computer have wireless Internet access over your phone) then Sprint will allow you to do that for free as long as you have an existing unlimited data plan ($15 for base plans or included in new plans), but Verizon will try to charge you per-megabyte costing you hundreds of dollars a month once they find out. Also be aware that Sprint also includes "Any Mobile-to-Mobile" add-on in many of their plans calling any of your friends on any mobile network completely free.
The iPhone is no longer a booster to AT&T's service since there are other alternative phones out or about to come out this year to rival the iPhone. T-Mobile is a company that has changed names three times already always hiding from a bad rep but trying to make money on we-cut-our-own-throat prices.
Some HTC phones that are are out already for Sprint and Verizon networks (both CDMA based) and are also GSM six-band phones so you have international usage, or additionally if they are Hard-SPL flashed, Secure Unlocked, and flashed with a custom WU World Unlocked Radio they can use US mobile carrier SIM cards letting you use AT&T, T-Mobile, or other carriers with regular or pre-paid SIM cards. You simply let the phone choose the network automatically by availability or manually by switching between CDMA and GSM only modes on the phone.
This CDMA & GSM access makes these phones almost universal in usage since they are carrier independent. On top of this you can flash them with tons of custom ROMs giving you access to all versions of Microsoft Windows Mobile 6.1 and 6.5 already including tons of custom applications written for these operating systems. You can even build your own custom ROMs from "kitchens" customizing the settings, drivers, and software available on these phones. They sell for $325-350 on eBay and can be activated with either carrier without a contract. Be sure to only by the Sprint or Verizon (CDMA & GSM enabled) HTC Touch Pro 2 phones and not the AT&T or T-Mobile (GSM only) since you won't have access to both wireless network types and the GSM only phones have a slower processor.
HTC Touch Pro 2 (aka RhodiumW) - 2 x CDMA, 6 x GSM, 480x800, 528MHz, 288/512 MB
Sprint - HTC Touch Pro 2
Verizon - HTC Touch Pro 2
Websites that you must visit.
If you can wait a bit longer and wish to spend $750 or more then you can consider this phone.
HTC HD2 or wait for the predicted but not confirmed HTC HD2 Pro (with keyboard and CDMA later this year).