Hoping there is a simpler way that I am missing, here is how to begin a new transaction with Doctrine 1.2 through Symfony 1.4. It was the call to sfDoctrineDatabase::getDoctrineConnection() which I had to lookup in the docs1.
$connection = sfContext::getInstance()->getDatabaseManager()->getDatabase('default')->getDoctrineConnection();
$connection->beginTransaction();
try {
// Do work for the transaction ...
$connection->commit();
} catch (Exception $e) {
$connection->rollback();
// Handle exception ...
}