How to calculate the 4 pairs of latitude and longitude that bound a city programmatically?

I've been researching ways to implement offline maps on iPhone using OpenStreetMap (with CloudMade tile server). I'd like to be able to allow users to request for offline maps based on a given city name (e.g. San Francisco, CA). The tile serv开发者_C百科er http interface suggests that I'd need to know the range of latitudes and longitudes covering the given city. Is this feasible? How do I calculate this programmatically?