added a FormExtension (works with the form branch of Symfony2)

Usage:

    $form = $app['form.factory']->createBuilder('form')
       ->add('first_name', 'text')
       ->add('last_name', 'text');

    return $app['twig']->render('form', array('form' => $form->getView()));

src/Silex/Extension/FormExtension.php

+<?php
+
+/*
+ * This file is part of the Silex framework.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Silex\Extension;
+
+use Silex\Application;
+use Silex\ExtensionInterface;
+use Symfony\Component\HttpFoundation\File\TemporaryStorage;
+use Symfony\Component\Form\CsrfProvider\DefaultCsrfProvider;
+use Symfony\Component\Form\Type\Loader\DefaultTypeLoader;
+use Symfony\Component\Form\FormFactory;
+
+class FormExtension implements ExtensionInterface
+{
+    public function register(Application $app)
+    {
+        $app['form.csrf.secret'] = '12345';
+        $app['form.storage.secret'] = 'abcdef';
+
+        $app['form.factory'] = $app->share(function () use ($app) {
+            return new FormFactory($app['form.type_loader']);
+        });
+
+        $app['form.type_loader'] = $app->share(function () use ($app) {
+            if (!isset($app['validator'])) {
+                throw new \RuntimeException(sprintf('The FormExtension needs the ValidationExtension to be registered.'));
+            }
+
+            return new DefaultTypeLoader($app['validator'], $app['form.csrf.provider'], $app['form.storage']);
+        });
+
+        $app['form.csrf.provider'] = $app->share(function () use ($app) {
+            return new DefaultCsrfProvider($app['form.csrf.secret']);
+        });
+
+        $app['form.storage'] = $app->share(function () use ($app) {
+            return new TemporaryStorage($app['form.storage.secret']);
+        });
+
+        if (isset($app['form.class_path'])) {
+            $app['autoloader']->registerNamespace('Symfony\\Component\\Form', $app['form.class_path']);
+        }
+    }
+}