>So I read, for example, that 'slightly more than one-half of the
>nation's population lived in jurisdictions --- cities, towns,
>boroughs, villages and townships --- with fewer than 25,000 people
>or in rural areas.
Sure. Like Avocado Heights, population just over 15,000 and another little piece of the megalopolis that is Los Angeles County:
Avocado Heights is an unincorporated census-designated place in Los Angeles County, California, United States. It is in the San Gabriel Valley near the Puente Hills. North of Avocado Heights is the unincorporated community of Bassett. To the east is the City of Industry. There is a very thin strip of the City of Industry to the south, with the Pomona Freeway and the Puente Hills further south. To the west is the San Gabriel River Freeway and the San Gabriel River. The California Country Club is also on the west side of Avocado Heights. Avocado Heights is approx. 15 miles from the downtown Los Angeles Civic Center. The population was 15,148 at the 2000 census.
There are many more like this:
http://en.wikipedia.org/wiki/Los_Angeles_County#Other_cities