It's not clear if an iphone or android app would be needed fo such a purpose. An html5 touch-accelerated page would be sufficient.

You know, NYC would be a nice place to start. You have the subway map anyway so you could reasonably estimate where people could live subject to commute constraints and actually see where the critical resources are located. The real innovation here would be in a nice UI -- using craigslist or going with a broker are pains in the arse.