[jQuery] jQuery $.get callback function not being called

[ArnieML]

Hello, I hope someone can help. I'm issuing the following ajax jquery
call:

        $.get("/ajaxtest.php", { ajax: "true", east: northEast.lng(), west:
southWest.lng(), north: northEast.lat(), south: southWest.lat(), mood:
amood },
        function (data) {
                for(var i=0; i<data.length; i++){
                alert("now processing tweets");
                display_tmmmarker(map, data[i], i);
                }
                $("#gettingtweets").remove();

        }, 'json');


The PHP page looks like this:

if ( $_REQUEST['ajax'] == 'true' ) {
        $east   = $_REQUEST['east'];
        $south  = $_REQUEST['south'];
        $west   = $_REQUEST['west'];
        $north  = $_REQUEST['north'];
        $mood   = $_REQUEST['mood'];

        $tweets = get_tmmtweets($east, $south, $west, $north, $mood);

        json_encode($tweets);
        }
exit;

I have verified that the $tweets array has been JSON-encoded using
array_values. But the callback function isn't being invoked. I put in
an alert statement in the callback to verify if the function has been
called, but I'm not getting the alert message. Is the $tweets array
not being passed on for some reason?

Can someone help out? Thanks very much!