As the train to New England takes me past a staccato skyline of tall buildings I remember the James Blish "Cities in Flight" series of 1950's science-fiction novels, and the transparent riddle that appears at one critical plot juncture: "What city has two names twice?"
Blish's Cities yarns grew out of a clever conceit: what if economies of scale made it possible to cheaply build spacecraft—via otherwise-unexplained "spindizzy" force-fields—that were several miles across? Blish postulated that whole towns could then surround themselves in bubbles of energy and set off through space. Adventures followed, on a cosmic and eventually meta-cosmic scale.
Don't ask how a metropolis could survive without upstream watersheds, air purification facilities, an agricultural base, and countless other critical ecosystem elements; those factors weren't part of the vision!
(see Andy Hooper's "Two Names Twice: James Blish and Cities in Flight" in the sf fanzine Chunga issue #5, August 2003; cf. LensManic (16 Jul 2001), SkylarkDuquesne (1 Nov 2003), CountermeasureAndGodshatter (30 Oct 2004), ...)
In a tangentially-related note, the band "Boston" had a theme with its album covers where Boston was shown as a city-spaceship.