I have created this simple three step approach for a Symfony project to switch application contexts while respecting the desire to have a global project-wide config file, vhost and default mod_rewrite configuration.  

1. /config.php:

define('URI_ROOT', '/project/web/');
define('MY_ENV', 'dev');
define('MY_DEBUG', true);

$apps = array(
              'default' => 'frontend',
              'admin' => 'backend'
              );

2. /myboot.php

// adjusts SERVER REQUEST URI value                                                                                                                                                    
// if necessary and sets current app constant                                                                                                                                          
function routeApp($apps)
{
  $alt_app = '';
  if(isset($_SERVER['REQUEST_URI']))
    {
      foreach($apps as $alias => $app)
        {
          if($alias != 'default')
            {
              $alias_pattern = URI_ROOT.$alias;
              if(strpos($_SERVER['REQUEST_URI'], $alias_pattern) === 0)
                {
                  $alias_preg_pattern = '/^'.str_replace('/',
                                               '\\/', $alias_pattern).'/';
                  $_SERVER['REQUEST_URI'] =
                             preg_replace($alias_preg_pattern, URI_ROOT.'',
                                                     $_SERVER['REQUEST_URI']);
                  $alt_app = $app;
                  break;
                }
            }
        }
    }

  if($alt_app != '')
    {
      define('MY_APP', $alt_app);
    }
  else
    {
      define('MY_APP', $apps['default']);
    }
}

3. '/web/index.php' (custom front controller.)

require('../config.php');
require('../myboot.php');

routeApp($apps);

require_once(dirname(__FILE__).'/../config/ProjectConfiguration.class.php');

$configuration = ProjectConfiguration::getApplicationConfiguration(MY_APP,
                                                              MY_ENV, MY_DEBUG);
sfContext::createInstance($configuration)->dispatch();

Please hit me up on twitter if you have any comments/questions. @mikejw