Second attempt to add readonly attribute to entity fields using a Field Widget for textfields

readonly.module

<?php

  /**
   * Implements hook_field_widget_info()
   */
  function readonly_field_widget_info(){
    return array(
        'readonly_textfield' => array(
        'label' => t('Text field (Read Only)'),
        'field types' => array('text'),
        'settings' => array('size' => 60),
        'behaviors' => array(
          'multiple values' => FIELD_BEHAVIOR_DEFAULT,
          'default value' => FIELD_BEHAVIOR_DEFAULT,
        ),
      ),
    );
  }

  /**
   * Implements hook_field_widget_form()
   */
  function readonly_field_widget_form(&$form, &$form_state, $field, $instance, $langcode, 
    $items, $delta, $element) {
    if ($instance['widget']['type'] == 'readonly_textfield') {
      $element += array(
        '#type' => 'textfield',
        '#default_value' => isset($items[$delta]['value']) ? $items[$delta]['value'] : NULL,
        '#attributes' => array('readonly' => 'readonly'),  
        );
    }
    return $element;
  }