--- E:\Projekte\das-oertchen.de\planning\Downloads\Modules\pingback-6.x-2.x-dev_11-Januar\pingback\pingback.module	2010-08-28 15:58:31.000000000 +-0100
+++ E:\Projekte\das-oertchen.de\realisation\drupal6\sites\all\modules\pingback\pingback.module	2011-02-17 12:46:34.000000000 +-0100
@@ -308,13 +308,21 @@
     'comment' => '[...] '. $excerpt .' [...]',
     'hostname' => ip_address(),
     'format' => variable_get('pingback_input_format', FILTER_FORMAT_DEFAULT),
     'name' => $title,
     'homepage' => $pagelinkedfrom,
   );
-  comment_save($edit);
+  
+  // Validate pingbacks and allow hooking this validation with hook_comment()
+  comment_validate($edit);
+
+  // Pingbacks with errors won´t be saved (return value FALSE)
+  if (!comment_save($edit)) {
+    return xmlrpc_server_error(0, t('Validation error.')); // not sure about the error code and message
+  }
+  
   /*
   //bypass the hiding in pingback_form_alter() because we want to use the input format
   $GLOBALS['pingback_bypass_format_hiding'] = TRUE;
   drupal_execute('comment_form', $edit, array());
   $GLOBALS['pingback_bypass_format_hiding'] = FALSE;
   watchdog('debug', print_r(form_get_errors(), TRUE));