Custom Validator not being called

FutureDate.php

<?php
namespace SimonB\CMSBundle\Validator\Constraints;

use Symfony\Component\Validator\Constraint;

/**
 * @author simon
 * @Annotation
 */
class FutureDate extends Constraint
{
    public $message = 'The date "%date%" must be in the future.';
}

FutureDateValidator.php

<?php
namespace SimonB\CMSBundle\Validator\Constraints;

use Symfony\Component\Validator\Constraint;
use Symfony\Component\Validator\ConstraintValidator;

/**
 * @author simon
 */
class FutureDateValidator extends ConstraintValidator
{
    public function validate($value, Constraint $constraint)
    {
        $this->context->addViolation("SIMON GOT SOMETHING WRONG!");
//         $now = new \DateTime();
//         if (! $value instanceof \DateTime)
//             $value = new \DateTime($value);
//         if ($value < $now) {
//             $this->context->addViolation($constraint->message, array('%date%' => $value));
//         }
    }
}

Tile.php

<?php

namespace SimonB\CMSBundle\Entity;

use Doctrine\Common\Collections\ArrayCollection;
use Doctrine\ORM\Mapping as ORM;
use Symfony\Component\Validator\Constraints as Assert;
use SimonB\CMSBundle\Validator\Constraints as SBAssert;

/**
 * @ORM\Table()
 * @ORM\Entity()
 */
class Tile
{
// <SNIP>
    /**
     * @var string $contentText
     * @ORM\Column(type="text", nullable=true)
     * @SBAssert\FutureDate
     */
    private $contentText;
// <SNIP>
}