1900 MHz will get you by in NYC. AT&T (due to all the mergers and such) has both 850 MHz and 1900 MHz running in NYC. However, 850 MHz is going to be more useful within the city – the lower the frequency, the better luck you’ll have getting reception inside and around large metal buildings.

You can use a tri-band phone – but it won’t be optimal service. Your only hope is that AT&T has stuck enough 1900 MHz towers around that it won’t matter.