The Nokia N8 supports WCDMA 850/900/1700/1900/2100. This means consumers in the USA can insert their AT&T or T-mobile SIM cards and enjoy fast 3G internet speeds. To confirm this works, I tested both SIM cards for a speed test. Take a look at the results.
Nokia N8 on AT&T
Nokia N8 on T-mobile
The results show that 3G works on both T-mobile USA and AT&T. It also tells me that I should use the T-mobile SIM card in my area because it is faster than AT&T. GSM World has a nice coverage list for you to check the frequencies in your country/region.