From what I've seen, most of the teams come from 3 specific areas. I should probably classify Michigan as its own area because there are so many teams from there, but i'll broaden it to the Great Lakes region. Then a lot of other teams come from the west coast in the CA and AZ backwards J spot reaching from Wa, to TX. Then a lot of other teams come from the east coast, is specific, New England and Florida.
For exacts, check the team list from FIRST,
http://www.usfirst.org/frc/map/FMPro...-max=all&-find