Jason Fox has a recent post to his site about using "Accept" headers in Zend Framework 2 apps to set the format of the return data from a request.
In this article I detail the process by which you can set up your controller actions in Zend Framework 2 to return either the default HTML, or JSON data depending on the "Accept Header" in the request. It incorporates changes related to a security update added since this very helpful article was written, and expands on some of the intricacies of making your web layer objects better "json providers."
His example uses a "ViewJsonStrategy" and the criteria to look for to determine which version to respond with (HTML or JSON) - the Accept header. It uses the JSON encoder/decoder instead of the built-in PHP one to he could use the included "toJson" method to customize the output of the JSON instead of just returning everything.