Index: wp-includes/taxonomy.php
===================================================================
--- wp-includes/taxonomy.php	(revision 28655)
+++ wp-includes/taxonomy.php	(working copy)
@@ -1385,6 +1385,8 @@
 		$orderby = 't.slug';
 	} else if ( 'term_group' == $_orderby ) {
 		$orderby = 't.term_group';
+	} else if ( 'post_date' == $_orderby ) {
+		$orderby = 'p.post_date';
 	} else if ( 'none' == $_orderby ) {
 		$orderby = '';
 	} elseif ( empty($_orderby) || 'id' == $_orderby ) {
@@ -1561,6 +1563,15 @@
 
 	$join = "INNER JOIN $wpdb->term_taxonomy AS tt ON t.term_id = tt.term_id";
 
+	// if we're ordering by date, we need to SELECT DISTINCT and JOIN term_relationships and posts
+	if ( 'post_date' == $_orderby ) {
+		$fields = "DISTINCT " . $fields;
+		// JOIN with wp_term_relationships
+		$join .= " JOIN $wpdb->term_relationships AS tr ON tr.term_taxonomy_id = tt.term_taxonomy_id";
+		// JOIN with wp_posts
+		$join .= " JOIN $wpdb->posts AS p ON p.ID = tr.object_id";
+	}
+
 	$pieces = array( 'fields', 'join', 'where', 'orderby', 'order', 'limits' );
 
 	/**