<?php
// Note: Change this line to switch which forum this block appears in.
$forum_id = 34;

if (arg(0) == 'forum' && arg(1) == $forum_id) {
  return TRUE;
}
if (arg(0) == 'node' && is_numeric(arg(1))) {
  $node = node_load(arg(1));
  if ($node->type == 'forum' && $node->tid == $forum_id) {
    return TRUE;
  }
}
return FALSE;
?>