Index: wp-includes/taxonomy.php
===================================================================
--- wp-includes/taxonomy.php	(revision 13594)
+++ wp-includes/taxonomy.php	(working copy)
@@ -233,14 +261,12 @@
 	}
 
 	if ( false !== $args['rewrite'] && '' != get_option('permalink_structure') ) {
-		if ( !is_array($args['rewrite']) )
-			$args['rewrite'] = array();
-		if ( !isset($args['rewrite']['slug']) )
-			$args['rewrite']['slug'] = sanitize_title_with_dashes($taxonomy);
-		if ( !isset($args['rewrite']['with_front']) )
-			$args['rewrite']['with_front'] = true;
+		$args['rewrite'] = wp_parse_args($args['rewrite'], array(
+			'slug' => sanitize_title_with_dashes($taxonomy),
+			'with_front' => true,
+		));
 		$wp_rewrite->add_rewrite_tag("%$taxonomy%", '([^/]+)', $args['query_var'] ? "{$args['query_var']}=" : "taxonomy=$taxonomy&term=$term");
-		$wp_rewrite->add_permastruct($taxonomy, "/{$args['rewrite']['slug']}/%$taxonomy%", $args['rewrite']['with_front']);
+		$wp_rewrite->add_permastruct($taxonomy, "{$args['rewrite']['slug']}/%$taxonomy%", $args['rewrite']['with_front']);
 	}
 
 	if ( is_null($args['show_ui']) )