In a new post to his blog Rob Allen shows how you can return JSON data directly back from a controller in a Zend Framework 2 application.

The new view layer in Zend Framework 2 can be set up to return JSON rather than rendered HTML relatively easily. [...] Firstly we need to set up the view's JsonStrategy to check to a situation when returning JSON is required and then to render out JSON for us.

This "JsonStrategy" does some of the hard work for you - detecting when the client is requesting a JSON response and looking at the data coming into the view to see if it's JSON. He shows how to implement it in a sample module using the "onBootstrap" module and how to force a return of the JsonModel even when JSON isn't requested (useful for a consistent interface).