Most unlocked/unbranded phones that are sold (through online shops or eBay) are triband but they're mostly the Euro version of a phone and have 900/1800/1900 bands.

Frequencies used in the US are 1900mhz (T-mobile) or 850 and 1900mhz(AT&T/Cingular).

If you're using AT&T, you want a phone that has both 850 and 1900mhz bands because I think that both frequencies are being used in your area.

If you get a phone that is triband 900/1800/1900 (meaning you're only using 1900) you might only get reception half of the time. Plus you won't get good reception when you're inside building as the 850 band has better penetration than the 1900 band.