With which carriers can I use an officially-unlocked iPhone?
You will find an up to date list of carriers and frequencies on this list for the United States. For other countries see here.

You can use your iPhones as follows (source):


iPhone Quad band GSM/GPRS/EDGE (850, 900, 1800, 1900 MHz)
iPhone 3G In addition to previous: Tri-band 3.6 Mbit/s UMTS/HSDPA (850, 1900, 2100 MHz)
iPhone 3GS In addition to previous: 7.2 Mbit/s HSDPA
iPhone 4 quad-band GSM/GPRS/EDGE (800 850 900 1800 1900 MHz). Quad-band UMTS/HSDPA/HSUPA (800 850 900 1900 2100 MHz) (800 MHz not yet officially supported by Apple)
iPhone 4S quad-band GSM/GPRS/EDGE (800 850 900 1800 1900 MHz) Quad-band UMTS/HSDPA/HSUPA (800 850 900 1900 2100 MHz) (800 MHz unannounced)


This means you can use your iPhones on any carrier on the 850, 900, 1800, 1900 MHz frequencies as a GSM phone, and access to data at the slow GPRS/EDGE speed.

For higher speed access you will have to look for carriers who provide the right technologies at the right frequencies.

This mean the iPhone will not work on networks operating in the 1700 MHz frequencies and it will not work on 1800MHz 3G (UMTS/HSDPA/HSUPA) networks as it only can use this band for GSM.