If you write an iPhone application that uses the network, then you must check if the network is both operating and that your server is reachable. Apple has and will continue to reject applications that do not test reachability. To this end, Apple provides iPhone developers a sample application and class, Reachability, that can easily be used to perform these tests

Three20 is a collection of iPhone UI classes, like a photo viewer, and general utilities, like an HTTP disk cache. Three20 is derived from the Facebook iPhone app.