format_username() was renamed to user_format_name().

modules/user/views_handler_field_user_name.inc

@@ -75,7 +75,7 @@ function render_link($data, $values) {
     }
     // If we want a formatted username, do that.
     if (!empty($this->options['format_username'])) {
-      return format_username($account);
+      return user_format_name($account);
     }
     // Otherwise, there's no special handling, so return the data directly.
     return $data;

modules/user/views_plugin_argument_validate_user.inc

@@ -122,7 +122,7 @@ function validate_argument($argument) {
     }
 
     $this->argument->argument = $account->uid;
-    $this->argument->validated_title = check_plain(format_username($account));
+    $this->argument->validated_title = check_plain(user_format_name($account));
     return TRUE;
   }