static flag to avoid loop

gistfile1.txt

wibble_node_presave($node) {
  if (node->type == 'whatever') {
    static $edit_flag;    
    if (reference_has_changed && empty($edit_flag)) {
      $edit_flag = TRUE;
      $reference = node_load($node->reference_field[0]['nid']);
      $reference->reference_field[0]['nid'] = node->nid;
      node_save($reference);
    }
  }
}