Answers: The iPhone is a quad group GSM device meaning that it can use adjectives of the GSM frequencies around the world. The frequencies are 800/1900 (US) and 900/1800 (Europe).
The iPhone is currently only is available beside AT&T and they use the 800 frequency (850 MHz), but as AT&T does have roaming agreements next to T-Mobile the 1900 MHz frequency is used as well.