[Freddie Bingham]

Change the references in the orderby section in forumdisplay.php from lastpost to dateline. This:

switch ($sortfield) {
  case 'title':
  case 'lastpost':
  case 'replycount':
  case 'views':
  case 'postusername':
  case 'voteavg':
    break;

  default:
    $sortfield='lastpost';
}
if ($sortfield=="voteavg" and !$foruminfo[allowratings]) {
  $sortfield="lastpost";
}

You should also edit the forumdisplay_threadslist template and change references from lastpost to dateline.

Something like that should work (or not)