Index: wp-admin/includes/post.php
===================================================================
--- wp-admin/includes/post.php	(revision 21499)
+++ wp-admin/includes/post.php	(working copy)
@@ -1340,5 +1340,5 @@
 		$url = add_query_arg( array( 'preview' => 'true', 'preview_id' => $id, 'preview_nonce' => $nonce ), get_permalink($id) );
 	}
 
-	return $url;
+	return apply_filters( 'preview_post_link', $url );
 }