$(function() {

if ($("#warning :checkbox").is(':checked'))
    $("form input[type='submit'][id$='dfSubmit']").removeAttr('disabled');
else
    $("form input[type='submit'][id$='dfSubmit']").attr("disabled", "disabled");
    
    $("#warning input[type='checkbox']").click(function() {
        if ($("#warning :checkbox").is(':checked'))
            $("form input[type='submit'][id$='dfSubmit']").removeAttr('disabled');
        else
            $("form input[type='submit'][id$='dfSubmit']").attr("disabled", "disabled");
    });
});