Controller.php

<?php

    protected function _editAction(Post $post)
    {
        $em = $this->get('doctrine.orm.default_entity_manager');
        $factory = $this->get('form.factory');
        $form = $factory->create(new PostFormType());
        $form->setData($post);

        if ($this->get('request')->getMethod() === 'POST') {
            $form->bindRequest($this->get('request'));

            if ($form->isValid()) {
                $em->persist($post);
                $em->flush();

                return new RedirectResponse($this->generateUrl('hello_index'));
            }
        }

        return $this->render('MyHelloBundle:Hello:edit.html.twig', array(
            'form' => $form->createView(),
        ));
    }

PostFormType.php

<?php

namespace My\HelloBundle\Form;

use Symfony\Component\Form\FormBuilder;
use Symfony\Component\Form\Type\AbstractType;
use My\HelloBundle\Entity\Comment;

class PostFormType extends AbstractType
{
    public function buildForm(FormBuilder $builder, array $options)
    {
        $builder
            ->add('title')
            ->add('file')
            ->add('content')
            ->add('abstract')
            ->add('enabled')
            ->add('publicationDateStart', 'date')
            ->add('commentsDefaultStatus', 'choice', array(
                'choices' => Comment::getStatusCodes(),
            ))
            // the second parameter, $type, is null because we use auto-creation
            ->add('tags', null, array(
                'expanded' => true,
                'multiple' => true,
            ))
            ->build('author', 'form', array('data_class' => 'My\HelloBundle\Entity\Author'))
                ->add('firstName')
                ->add('lastName')
            ->end();
    }

    public function getDefaultOptions(array $options)
    {
        return array(
            'data_class' => 'My\HelloBundle\Entity\Post',
        );
    }

    public function getName()
    {
        return 'postform';
    }
}

template.html.twig

{% form_theme form _self %}
{{ form_enctype(form) }}
{{ form_widget(form) }}
{{ form_widget(form.firstName) }}

{% for child in form %}
    {{ form_widget(child) }}
{% endfor %}

{{ form_widget(form.firstName, { 'attr': { 'class': 'foobar' } }) }}
{{ form_label(form.firstName, 'My label') }}

I forgot to say that your domain objects must implement the ToIdTransformable interface:

interface ToIdTransformable
{
    function getId();
}

But I really would like to have the thoughts of @bschussek on this :)