Workaround for Issue #2059 in Symfony 2.0

gistfile1.php

<?php
namespace Rousseau\PaymentBundle\Form\DataTransformer;

use Symfony\Component\Form\DataTransformerInterface;
use Symfony\Component\Form\Exception\TransformationFailedException;
use Doctrine\Common\Persistence\ObjectManager;

/*
 Usage:
   $builder->add(
                $builder->create('amount')->appendClientTransformer(new CommaToDotTransformer())
            );
*/

/* Allow using comma, when only dot is accepted by locale as decimal separator */
class CommaToDotTransformer implements DataTransformerInterface
{
    public function transform($number)
    {
        return $number;
    }

    public function reverseTransform($input)
    {
        return str_replace(',', '.', $input);
    }
}

From symfony 2.1, appendClientTransformer is replaced by addViewTransformer

->add(
    $builder
        ->create('your_field', 'text')
        ->addViewTransformer(new CommaToDotTransformer())
)

Very thank you