ContainerAwareInterface.php

<?php

namespace Symfony\Component\DependencyInjection;

/*
 * This file is part of the Symfony framework.
 *
 * (c) Fabien Potencier <[email protected]>
 *
 * This source file is subject to the MIT license that is bundled
 * with this source code in the file LICENSE.
 */

/**
 * ContainerAwareInterface should be implemented by classes that depends on a Container.
 *
 * @author Fabien Potencier <[email protected]>
 */
interface ContainerAwareInterface
{
    /**
     * Sets the Container.
     *
     * @param ContainerInterface $container A ContainerInterface instance
     */
    function setContainer(ContainerInterface $container = null);
}

controllers.xml

<?xml version="1.0" ?>

<container xmlns="http://www.symfony-project.org/schema/dic/services"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://www.symfony-project.org/schema/dic/services http://www.symfony-project.org/schema/dic/services/services-1.0.xsd">

    <services>
        <service id="index_controller" class="Company\ApplicationBundle\Controller\SimpleController" />
    </services>
</container>

interfaces.xml

<?xml version="1.0" ?>

<container xmlns="http://www.symfony-project.org/schema/dic/services"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://www.symfony-project.org/schema/dic/services http://www.symfony-project.org/schema/dic/services/services-1.0.xsd">
  
    <interfaces>
        <interface class="Symfony\Component\DependencyInjection\ContainerAwareInterface">
            <call method="setContainer">
                <argument type="service" id="container" />
            </call>
        </interface>
    </interfaces>
</container>

SimpleController.php

<?php

namespace Company\ApplicationBundle\Controller;

use Symfony\Component\DependencyInjection\Container;
use Symfony\Component\DependencyInjection\ContainerAwareInterface;

class SimpleController implements ContainerAwareInterface
{
    /**
     * @var Symfony\Component\DependencyInjection\Container
     */
    private $container;

    /**
     * @param Symfony\Component\DependencyInjection\Container $container
     */
    public function setContainer(Container $container)
    {
        $this->container = $container;
    }
    //...
}

This is kinda awesome if i understand it correctly. So if a class implementes a specific interface (in this case ContainerAwareInterface) the service will call setContainer for that class?

yes, you can read more here http://avalanche123.com/post/1221451286/interface-injection-and-symfony2-dic