if you want only admins and mods to see hidden text without replying:
For SMF 1.1 RC 1 only
Find in Sources/Subs.php
$codes = array(
add after
array(
'tag' => 'hide',
'type' => 'unparsed_content',
'content' => '<b>$1</b>',
'validate' => create_function('&$tag, &$data, $disabled', '
global $context, $ID_MEMBER, $db_prefix, $user_info, $txt;
if (!empty($context[\'current_topic\']) && !$user_info[\'is_guest\'])
{
$request = mysql_query("
SELECT ID_MSG FROM {$db_prefix}messages
WHERE ID_MEMBER = $ID_MEMBER
AND ID_TOPIC = $context[current_topic]
LIMIT 1");
if (mysql_num_rows($request) || allowedTo(\'moderate_forum\'))
$data;
else
$data = $txt[\'hide_tag_a\'];
mysql_free_result($request);
}
elseif ($user_info[\'is_guest\'])
{
$data = $txt[\'hide_tag_b\'];
}
$cache_id = "";
'),
),
note that the only drawback to this is that to admins and mods, the content between the hidden tags will display as unparsed.