I'm designing the back-end for an iphone application. I'd like to use Core Data. Is there an ideal transfer format for iphone apps? I'm leaning towards JSON unless there's some custom binary thing. I'm probably using ruby on the backend.
For example, in the Flex/Flash world, you can install a small piece on your server that lets you send and receive strongly-typed objects - flash and your server piece take care of all the serialization and hydrating.
JSON or XML has worked best for me in the past. There are some formats that the iPhone SDK handles natively (plist), but I'd avoid those as you may want to open up your service to other clients at later time.
The iPhone SDK includes a SAX parser (NSXMLParser), but you can freely import other libraries like libXML if you like DOM better. SAX fits a little better in low memory spaces though.
For JSON you'll need a third party lib... I think SimpleJSON is one.