How can I change the delete message "This action cannot be undone." in a specific content type?
Try this code (it works for me - Drupal 6):

function module_change_default_messages_form_alter(&amp;$form, $form_state, $form_id) {
if (arg(0) == 'node' &amp;&amp; is_numeric(arg(1)) &amp;&amp; arg(2) == 'delete') {
      $node = node_load(arg(1));   
          if ($form_id == 'node_delete_confirm') {
              if($node-&gt;type == 'story'){
                $form['description'] = array('#value' =&gt; "here replace to \"this action cannot be undone\"");
              } else if($node-&gt;type == 'page'){
                $form['description'] = array('#value' =&gt; "this action cannot be undone");
              }
          }
      }
}


or you can use the PHP switch statement for more content types.