How do I find out what frequencies an iPhone supports?
See iPhone 3GS - Technical Specifications:

Cellular and wireless


UMTS/HSDPA (850, 1900, 2100 MHz)
GSM/EDGE (850, 900, 1800, 1900 MHz)
802.11b/g Wi-Fi
Bluetooth 2.1 + EDR wireless technology


From Wikipedia:


  In North America, GSM operates on the primary mobile communication bands 850 MHz and 1,900 MHz. In Canada, GSM-1900 is the primary band used in urban areas with 850 as a backup, and GSM-850 being the primary rural band. In the United States, regulatory requirements determine which area can use which band.