One problem with the iPhone in the US is that users are beholden to the mega-carriers and all their assorted issues and requirements. In particular, the GSM iPhone sold locked through AT&T or available factory-unlocked from Apple requires AT&T service to get 3G data. The iPhone will technically work on T-Mobile, but only over EDGE, not 3G. This is because T-Mobile supports 3G over the 1700/2100 MHz bands, with 850 MHz used for GSM/GPRS/EDGE connections. AT&T uses 850/1900 MHz for 3G, and 850 MHz for GSM/GPRS/EDGE connections. However, a smaller carrier named H2O Wireless, which uses the same 3G bands as AT&T, is now offering support for unlocked phones that support those wireless bands, including the iPhone.
H2O Wireless offers contract-free wireless plans, including data plans for smartphones, with an unlimited data plan available for $60. This includes plans for phones that would hypothetically work on unlocked GSM phones, such as the iPhone and Android phones, as H2O offers just a SIM card that can be used in these GSM phones. Now, these phones have to be configured to access H2O Wireless' network, but this can be done relatively easily. For iPhone users, they have to install a configuration file that will enable the phone to be used on their network. Android users with phones that support the 850/1900 MHz 3G bands can just download an app from the Android Market to update their wireless settings. H2O Wireless has an automatic setup app that will work on iPhones, but if Apple ever approved it for the App Store, fire would rain from the skies and the end of days would be upon us.
H2O Wireless doesn't just purport to be some dinky local carrier, either - their service map encompasses many portions of the US with major metropolitan areas. For users with GSM iPhones that want to avoid contracts and/or AT&T's service issues, this smaller carrier may be an option.
Update: The MyH2O app was approved by Apple recently, allowing for users to configure their phones to use H2O Wireless, as well as giving the ability to manage their account directly from the app. If fire rains from the skies and the end of all days does indeed happen, then I apologize profusely. If it doesn't, then just ignore me the next time I make these dire predictions about the end of mankind in articles on an iOS website!