AT&T & Rogers: 850/1900 MHz
T-Mobile 1700/2100 MHz (not compatible with normal 2100 MHz)
Telstra (Australia) 850/2100 MHz.
Everybody else: 2100 MHz.
Most 3G phones use either a 2100MHz chipset or a 850/1900MHz chipset. Note that that leaves both T-Mobile and Telstra somewhat out in the cold, although tri-band chips will bring Telstra back in.
Speaking of tri-band chips they do exist, but in size and power consumption are not yet ready for prime time.
Hence any current 3G iPhone would require at least two models: one for Europe, one for North America (and likely one for Japan, given market requirements, although they get Rev. B of the European 3G iPhone).
I stand by my thoughts that a European 3G iPhone in October and a North American 3G iPhone in late November (FCC approval lag) would be the best way to go in terms of long-term strategy.
Apple recently signed a deal with InterDigital, who are most notably not Qualcomm.