Marginal Revolutions comes this link to Strange Maps, which has an entry addressing the number and locations of all of the McDonalds in the United States. According to the site, the farthest point from the closest McDonalds is somewhere in rural South Dakota:
There are over 13,000 McDonald’s restaurants in the US, or about 1 for every 23,000 Americans. But even market penetration this advanced doesn’t mean that McDonald’s is everywhere. Somewhere in South Dakota is the McFarthest Spot, the place in the US geographically most removed from the nearest McD’s (*). If you started out from this location, a few miles north of State Highway 20 (which runs latitudinally between Highways 73 in the west and 65 in the east), you’d have to drive 145 miles to get your Big Mac (if you could fly, however, it’d be only 107 miles).
I also like the post's discussion of how closely the map of the various McDonald's locations mirrors that of the population at large. Check out the link simply for the map, but here is the last paragraph:
Out west, the Arches are fewer and further between, with the exception of the heavily populated coastal areas. To achieve identical density to the rest of the country, this sparsely burgered part of the country would have to be sandwiched between them so that southern California and western Texas would almost touch, and Seattle would be a day’s drive from Minneapolis. The blackest holes in the western McTapestry are the Nevada desert, some mountainous parts of Oregon and Idaho, and the plains of South Dakota – home to the aforementioned McFarthest Spot.
Interesting post, and interesting site, especially if, like me, you enjoy maps.