While wandering around on the Web tonight, I ran across this Walking Cities project by Ron Herron (a member of the Archigram avantgarde group). In this concept, the city is a giant, reptilian structures which glided across the globe until its inhabitants found a place where they wanted to settle.

It seems that this concept is now forgotten, there are still temporary architectures but moving habitations (apart from trailers) are not really trendy...