Index: wp-includes/user.php
===================================================================
--- wp-includes/user.php	(revision 18495)
+++ wp-includes/user.php	(working copy)
@@ -432,6 +432,10 @@
 			$orderby = $qv['orderby'];
 		} elseif ( 'name' == $qv['orderby'] || 'display_name' == $qv['orderby'] ) {
 			$orderby = 'display_name';
+		} elseif( !empty($qv['meta_key']) && 'meta_value' == $qv['orderby'] ) {
+			$orderby = "$wpdb->usermeta.meta_value";
+		} elseif( !empty($qv['meta_key']) && 'meta_value_num' == $qv['orderby'] ) {
+			$orderby = "$wpdb->usermeta.meta_value+0";
 		} elseif ( 'post_count' == $qv['orderby'] ) {
 			// todo: avoid the JOIN
 			$where = get_posts_by_author_sql('post');