Let the hive mind of Engadget get that for you.
"I just switched to Sprint from Verizon about three months ago for the Pre. Then I went for the Hero about a week ago. Now, I miss my hardware keyboard and am thinking about switching to the Moment. I am still able to switch back to Verizon if I want and get the Droid when it arrives. Should I just trade up to the Moment when it comes out, see if I like it, and if not switch to the Droid? Or something else entirely? Help!"
The short answer k-y is that AT&T, the largest GSM provider in the US, uses the 850 and 1900 mhz bands, with predominant usage of the 850 band in most areas. Tri-band phones support 900/1800/1900, but lack the 850 band.
The same is true for UMTS/HSDPA. AT&T's currently deployed 3G network relies on some oddball (compared to the rest of the world) bands.