Your phone’s GSM-1900 band will work in the US, but rural coverage may be lacking.
Listing of US GSM companies with coverage maps, from the GSM Assocation. Most of these seem to small local companies that may not have roaming agreements.
The big US GSM companies include Cingular and T-Mobile. (I could have sworn AT&T was in there somewhere, but the corporations ave merged and unmerged and renamed themselves and split and bought each other up for so long that I have totally lost count of who’s who anymore.)
More info…
GSM frequency ranges, from Wikipedia.
My cellphone company’s GSM frequency-usage maps of the world.
The Americas are a patchwork of all four GSM cellphone frequencies.
GSM-1900 is common in the US and Canada, and also scattered other parts of the Americas that have been under US technical influence. The US, Canada, and several other countries also use GSM-850, which is replacing the GSM carriers’ legacy analogue cellphones in rural areas.
A few American countries use only GSM-850: Panama, for instance.
Areas under ‘European’ influence, such as the French overseas territories, tend to use the mainline GSM frequencies (GSM-900 and 1800).
Brazil uses GSM-1800. Venezuela uses GSM-900. Peru uses 1900 and 850, I think.
Some Caribbean islands use a mixture of parts of all four standards. I’m still trying to figure that one out, especially since GSM850 overlaps GSM-900 and GSM-1900 overlaps GSM-1800.
I avoided the whole problem and bought a quad-band phone. 