Index: wp-includes/taxonomy.php
===================================================================
--- wp-includes/taxonomy.php	(revision 23382)
+++ wp-includes/taxonomy.php	(working copy)
@@ -2056,6 +2056,14 @@
 
 	$defaults = array( 'alias_of' => '', 'description' => '', 'parent' => 0, 'slug' => '');
 	$args = wp_parse_args($args, $defaults);
+
+	// 'parent' argument could be a slug too
+	if ( ! is_numeric( $args['parent'] ) ) {
+		$term_parent = get_term_by( 'slug', $args['parent'], $taxonomy );
+		if ( $term_parent )
+			$args['parent'] = $term_parent->term_id;
+	}
+
 	$args['name'] = $term;
 	$args['taxonomy'] = $taxonomy;
 	$args = sanitize_term($args, $taxonomy, 'db');
@@ -2368,6 +2376,14 @@
 
 	$defaults = array( 'alias_of' => '', 'description' => '', 'parent' => 0, 'slug' => '');
 	$args = wp_parse_args($args, $defaults);
+
+	// 'parent' argument could be a slug too
+	if ( ! is_numeric( $args['parent'] ) ) {
+		$term_parent = get_term_by( 'slug', $args['parent'], $taxonomy );
+		if ( $term_parent )
+			$args['parent'] = $term_parent->term_id;
+	}
+
 	$args = sanitize_term($args, $taxonomy, 'db');
 	extract($args, EXTR_SKIP);