There are two types of floating cities: cities that float in water (see ocean colonization) and cities that float in the sky (such as Cloud City). Both are still theoretical; the latter is really theoretical.
Geoffrey Landis provides an interesting point though about building floating cities on Venus, whose surface is generally considered to be akin Hell:
However, viewed in a different way, the problem with Venus is merely that the ground level is too far below the one atmosphere level. At cloud-top level, Venus is the paradise planet.
(quote from Wikipedia)
There was a suggestion somewhere that if you built a geodesic dome of a city size, the heating of the air within would make it float. link
This is great! The Cloud Nine concept is a nice extension of the geodesic dome.
Floating “cities” may be more a reality than one would think. There are a few dozen permanently inhabited floating islands created by the Uros people of reeds on lake titicaca between the peruvian and bolivian borders. However, I wouldn’t exactly call these isles urban centers in the traditional sense:
http://en.wikipedia.org/wiki/Uros