ncux, как то так
$arrids = array();
$sql = mysql_query("select `id` from `table`");
while ($row = mysql_fetch_row($sql)) {
$arrids[] = $row[0];
}
$array = range(1, 1000);
$result = array_diff($arrids, $array);
echo 'Свободны номера ' . implode(' ', $result);