I've created new symfony 1.4.8 project:

$ ./symfony -V
symfony version 1.4.8 (/home/marcin.dryka/htdocs/leak/lib/vendor/symfony/lib)

$ php -v
PHP 5.3.2-1ubuntu4.5 with Suhosin-Patch (cli) (built: Sep 17 2010 13:41:55)
Copyright (c) 1997-2009 The PHP Group
Zend Engine v2.3.0, Copyright (c) 1998-2010 Zend Technologies
with Xdebug v2.1.0, Copyright (c) 2002-2010, by Derick Rethans

and set the database schema as follows:

$ cat config/doctrine/schema.yml
Example:
columns:
col1: string(255)
col2: string(255)
col3: string(255)
col4: string(255)
col5: string(255)
col6: string(255)

I created a task that contains a Doctrine_query call

(...)
protected function execute($arguments = array(), $options = array())
{
// initialize the database connection
$databaseManager = new sfDatabaseManager($this->configuration);
$connection = $databaseManager->getDatabase($options['connection'])->getConnection();

while(1)
{
$m = xdebug_memory_usage();
$q = Doctrine_Query::create()
->from('Example');
$o = $q->fetchOne();

if (false !== $o))

unset($q, $o);

printf("Delta: %s Value: %s\n",
xdebug_memory_usage()-$m,
xdebug_memory_usage()
);
}
}
(...)

Unfortunately, memory usage is increasing:
./symfony leak
Delta: 3285264 Value: 10651596
Delta: 12944 Value: 10664448
Delta: 12952 Value: 10677308
Delta: 12932 Value: 10690148
Delta: 12932 Value: 10702988
Delta: 12932 Value: 10715828
Delta: 12932 Value: 10728668
Delta: 12932 Value: 10741508
Delta: 12932 Value: 10754348
Delta: 12932 Value: 10767188

Tested with and without data in database - result is the same.