--- modules/node/node (copy).pages.inc	2008-07-11 21:05:56.000000000 +0700
+++ modules/node/node.pages.inc	2008-07-11 22:12:53.000000000 +0700
@@ -160,6 +160,7 @@ function node_form(&$form_state, $node) 
       '#type' => 'textarea',
       '#title' => t('Log message'),
       '#rows' => 2,
+      '#default_value' => isset($form_state['node_preview']) ? ($node->log ? $node->log : '') : '',
       '#description' => t('An explanation of the additions or updates being made to help other authors understand your motivations.'),
     );
   }