Working with rjkha on http://drupal.org/node/626546

node_add_body_field

/**
 * Adds the default body field to a node type.
 *
 * @param $type
 *   A node type object.
 * @param bool $subject
 *   (optional) Include a summary field. Defaults to TRUE.
 * @param $label
 *   (optional) The label for the body instance.
 *
 * @return
 *   Body field instance.
 */
function node_add_body_field($type, $summary = TRUE, $label = 'Body') {
   // Add or remove the body field, as needed.
  $field = field_info_field('body');
  $instance = field_info_instance('node', 'body', $type->type);
  if (empty($field)) {
    $field = array(
      'field_name' => 'body',
      'type' => $summary ? 'text_with_summary' : 'text_long',
      'entity_types' => array('node'),
    );
    $field = field_create_field($field);
  }
  if (empty($instance)) {
    $instance = array(
      'field_name' => 'body',
      'entity_type' => 'node',
      'bundle' => $type->type,
      'label' => $label,
      'widget' => array('type' => $summary ? 'text_textarea_with_summary' : 'text_textarea'),
      'settings' => array('display_summary' => TRUE),
    );
    $instance = field_create_instance($instance);

    // Assign display settings for the 'default' and 'teaser' view modes.
    entity_get_display('node', $type->type, 'default')
      ->setComponent($field['field_name'], array(
        'label' => 'hidden',
        'type' => 'text_default',
      ))
      ->save();
    entity_get_display('node', $type->type, 'teaser')
      ->setComponent($field['field_name'], array(
        'label' => 'hidden',
        'type' => 'text_summary_or_trimmed',
      ))
      ->save();
  }

  return $instance;
}