Auto register 'category' for any custom post type

Adding the following in add_meta_boxes enables the category taxonomy for any custom post type.

foreach(get_post_types() as $post_type) {
  $exclude = array("wp-types-group", "wp-types-user-group", "post", "page", "attachment", "revision", "nav_menu_item");
  if ( in_array($post_type, $exclude) )
    continue;
  if (!is_object_in_taxonomy($post_type, 'category')){
    register_taxonomy_for_object_type('category', $post_type);
  }
}

http://wordpress.org/plugins/minimu/