Index: wp-includes/plugin.php
===================================================================
--- wp-includes/plugin.php	(revision 15828)
+++ wp-includes/plugin.php	(working copy)
@@ -555,7 +555,7 @@
 	$mu_plugin_dir = preg_replace('|/+|','/', $mu_plugin_dir); // remove any duplicate slash
 	$file = preg_replace('#^' . preg_quote($plugin_dir, '#') . '/|^' . preg_quote($mu_plugin_dir, '#') . '/#','',$file); // get relative path from plugins dir
 	$file = trim($file, '/');
-	return $file;
+	return apply_filters('plugin_basename', $file);
 }
 
 /**