JSON encoding with Services_JSON and json_encode in PHP returning empty object with jQuery ajax call

            $.ajax({
                url: '/ajax/get_rule.php',
                dataType: 'json',
                data: {                 
                    feature : feature,
                    ajax : 1
                },
                success: function(resp) {
                    console.log('in here');
                    console.log('response: ' + resp);

                },
                error : function() {
                    console.log('there was an error...');
                }
            }); 

I am using the PHP PEAR Services_JSON class to encode my array into a JSON string and then echo that out in the response, like this:

echo Registry('json')->encode(array('ajax' => 1, 'feature' => 'rfc1872'));
exit;

In the above example Registry('json') is a Services_JSON object.

I have also tried the PHP built-in function json_encode:

echo json_encode(array('ajax' => 1, 'feature' => 'rfc1872'));
exit;       

Neither of these work and although I don't fall into the error callback in the jQuery ajax call above I get a response value of response: [object Object] with an empty object in Firebug.

Why would I not receive a response back to my jQuery code with a JSON-encoded string?