hrm... actually, I'm reminded of something. Several years back, someone designed these simulations that basically ran an algorithm like "assume people don't mind being around people that are different, so long as at least some small fraction of their nearest neighbors are also like themselves.", and basically simulated people moving around to fulfil those criteria.

The simulation would consistently produce highly segregated results. Aha! here's a site with applets that run such simulations: http://www.dartmouth.edu/~segregation/segregation-simulator.html