Index: taxonomy.php
===================================================================
--- taxonomy.php	(revision 7598)
+++ taxonomy.php	(working copy)
@@ -743,9 +743,13 @@
 			return 0;
 		$where = $wpdb->prepare( "t.term_id = %d", $term );
 	} else {
-		if ( '' === $term = sanitize_title($term) )
+		if ( '' === $sanitized = sanitize_title($term) )
 			return 0;
-		$where = $wpdb->prepare( "t.slug = %s", $term );
+		if ( 'post_tag' === $taxonomy ) {
+			$where = $wpdb->prepare( "(t.slug = %s OR t.name = %s)", $sanitized , $term );
+		} else {
+			$where = $wpdb->prepare( "t.slug = %s", $sanitized );
+		}
 	}
 
 	if ( !empty($taxonomy) )