// config/ProjectConfiguration.class.php
class ProjectConfiguration extends sfProjectConfiguration
{
  public function setup()
  {
    // ...

    $this->dispatcher->connect('request.filter_parameters', array($this, 'filterRequestParameters'));
  }

  public function filterRequestParameters(sfEvent $event, $parameters)
  {
    $request = $event->getSubject();

    if (preg_match('#Mobile/.+Safari#i', $request->getHttpHeader('User-Agent')))
    {
      $request->setRequestFormat('iphone');
    }

    return $parameters;
  }
}

<?php

class iPhoneFilter extends sfFilter
{
  public function execute ($filterChain)
  {
    // get the cool stuff
    $context    = $this->getContext();
    $request    = $context->getRequest();
    $response   = $context->getResponse();

    if ($this->isFirstCall())
    {
      $cookies = $response->getCookies();
      if (preg_match('#Mobile/.+Safari#i', $request->getHttpHeader('User-Agent'))
       && ! isset($cookies['doaction']))
      {
        $request->setRequestFormat('iphone');
      }
    }

    // execute next filter
    $filterChain->execute();
  }
}