Using ENTER key to submit default Drupal 7 Search doesn't not return results: Why?
Are you using the &lt;form&gt;'s submit event rather than the &lt;input&gt;'s click event to respond to the form submission? Check out this working example on jsFiddle.

$("#form").bind('submit', function(e) {
    var $div = $("&lt;div&gt;Transmitting&lt;/div&gt;").appendTo('body');
    $.ajax({
        url: '/ajax_html_echo/',
        data: {html: $("#html").val() },
        type: 'POST',
        success: function(data) {
            $div.html("Received: "+data);
        }
    });
    return false;
});