To fetch a random user:

$userCount = Doctrine::getTable('User')->count();
$user = Doctrine::getTable('User')
  ->createQuery()
  ->limit(1)
  ->offset(rand(0, $userCount - 1))
  ->fetchOne();
 

More a PHP trick than symfony's one, but as you can't with propel randomize order of results, just do:

$c = new Criteria()
... fill your criteria there ...
$result = MyTablePeer::doSelect($c);
shuffle($result);

obvious? sorry, seen someone asking on IRC once :D