Index: wp-includes/comment.php
===================================================================
--- wp-includes/comment.php	(revision 12320)
+++ wp-includes/comment.php	(working copy)
@@ -1675,6 +1675,7 @@
 			$client = new IXR_Client($pingback_server_url);
 			$client->timeout = 3;
 			$client->useragent .= ' -- WordPress/' . $wp_version;
+			$client->useragent = apply_filters('pingback_useragent', $client->useragent, $pingback_server_url, $pagelinkedto, $pagelinkedfrom);
 
 			// when set to true, this outputs debug messages by itself
 			$client->debug = false;