I\'ve started my degree project, a mobile application suitable for iPhone, Android and (in the near future) Symbian. The server architecture is the following:
If your looking for a language and platform agnostic solution have a look at Google Buffers. You can easily serialise objects for transmission over the wire.
This question should get you started in Objective-C.
I have also used JSON within iPhone Apps with great success. Again, relatively language and platform agnostic but much simpler than Google Buffers.