$this->addElement('password', 'password', 'Password:', array('id'=>'id_password'));
    $this->addElement('checkbox', 'unmask', 'Unmask password:', '', array('onclick'=>'togglePasswordMask('id_password')', 'id'=>'id_passwordunmask'));

$this->addElement('static', null, '',
            '<script type="text/javascript">
            //<![CDATA[
            function togglePasswordMask(id) {
                var pw = document.getElementById(id);
                var chb = document.getElementById(id+'unmask');
                var newpw = document.createElement('input');

                newpw.setAttribute('name', pw.name);

                if (chb.checked) {
                    newpw.setAttribute('type', 'text');
                } else {
                    newpw.setAttribute('type', 'password');
                }

                newpw.id = pw.id;
                newpw.value = pw.value;
                pw.parentNode.replaceChild(newpw, pw);
            }
            //]]>
            </script>');