Index: wp-includes/link-template.php
===================================================================
--- wp-includes/link-template.php	(revision 10864)
+++ wp-includes/link-template.php	(working copy)
@@ -88,6 +88,7 @@
 		$leavename? '' : '%postname%',
 		'%post_id%',
 		'%category%',
+		'%tag%',
 		'%author%',
 		$leavename? '' : '%pagename%',
 	);
@@ -129,6 +130,15 @@
 			}
 		}
 
+		$tag = '';
+		if ( strpos($permalink, '%tag%') !== false ) {
+			$tags = get_the_tags($post->ID);
+			if ( !empty( $tags ) ) {
+				usort($tags, '_usort_terms_by_ID'); // order by ID
+				$tag = $tags[0]->slug;
+			}
+		}
+
 		$author = '';
 		if ( strpos($permalink, '%author%') !== false ) {
 			$authordata = get_userdata($post->post_author);
@@ -147,6 +157,7 @@
 			$post->post_name,
 			$post->ID,
 			$category,
+			$tag,
 			$author,
 			$post->post_name,
 		);