I get most of my geological information from the wonderful tome written by John McPhee,
Annals of the Former World.
He outlines that most geothermal areas are near a transform fault, where one tectonic plate butts up against another one (say the Pacific Plate ramming into the North American Plate). These produce the "Ring of Fire" where most of the volcanoes occur. Therefore, most of the geothermal areas in the U.S. are in California, Oregon, Washington, Idaho.
There's another interesting thing that McPhee mentions: "Hot Spots" that exist intra-plate. There are only a few of these in N. America: Yellowstone, of course, but also Jemez Springs in New Mexico, and there is one in British Columbia.