The mail has a plain text, and HTML part to it. It generates the contents of the mail by rendering a partial, which requires a context to be set up. I modify sf_format by changing the request format on the request.

class nospOverusagereportTask extends sfBaseTask
{
  protected function configure()
  {
    $this->addOptions(array(
      new sfCommandOption('application', null, sfCommandOption::PARAMETER_REQUIRED, 'The application name', 'frontend'),
      new sfCommandOption('env', null, sfCommandOption::PARAMETER_REQUIRED, 'The environment', 'dev'),
      new sfCommandOption('connection', null, sfCommandOption::PARAMETER_REQUIRED, 'The connection name', 'radius'),
    ));

    $this->namespace        = 'nosp';
    $this->name             = 'over-usage-report';
    $this->briefDescription = '';
    $this->detailedDescription = 'moooo :)';
  }

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

    $form = new RadiusOverUsageForm();
    $form->bind(array(
      'percent' => 125,
      'payg'    => true,
    ));
    $results = $form->getResults();

    // create a context, and load the helper
    $context = sfContext::createInstance($this->configuration);
    $this->configuration->loadHelpers('Partial');

    // create the message
    $message = $this->getMailer()->compose('no-reply@domain.com', 'me@domain.com', 'Subject Line');

    // generate HTML part
    $context->getRequest()->setRequestFormat('html');
    $html  = get_partial('radius/overusage', array('results' => $results));
    $message->setBody($html, 'text/html');

    // generate plain text part
    $context->getRequest()->setRequestFormat('txt');
    $plain = get_partial('radius/overusage', array('results' => $results));
    $message->addPart($plain, 'text/plain');

    // send the message
    $this->getMailer()->send($message);
  }
}