Here’s something to go through before you start architecting your next massive online data access project:
Integrating iOS Applications with Backend REST Services
… This project showcases different transport formats and libraries to consume web services from an iPhone application. It features a server application written in PHP, a command-line script that saves the output of the server application in different formats, and an iPhone client application (compatible with iPhone OS 3.0 and higher). The server application reads a MySQL database and outputs data in the following formats: HTML, JSON, YAML, XML (arbitrary format), SOAP, Property list (Binary and XML), CSV, and Protocol Buffers…
Good stuff, good stuff. That should cover pretty much anything anybody wants to try. Project is on github: be sure to scroll down to the bottom of the readme for a good set of references, comparisons, and miscellaneous links!