We are using the Java Play framework for the REST API, MongoDB as our DB. MongoDB has geospatial index support that makes geo queries pretty efficient. For iOS, pretty standard Cocoa libraries... glad you think its responsive :) On Android, also pretty standard libraries.
We designed the client to be very light and do as much processing on the server and also are very careful to do as much asynchronous work as possible (networking a must)
We designed the client to be very light and do as much processing on the server and also are very careful to do as much asynchronous work as possible (networking a must)