I searched for a easy way to get values that contain a not null in the where clause. That's what I came up with:

$repo = $this->getDoctrine()->getRepository('LiplexBundle:User');
$query = $repo
    ->createQueryBuilder('u')
    ->where('u.lastOnline > :lastOnline')
    ->andWhere('u.posts IS NOT NULL')
    ->setParameter('lastOnline', new \DateTime('2 days ago'))
    ->getQuery();

There is also a isNotNull() expression, but the only thing it does is returning a concatenated string with IS NOT NULL:

/**
 * Creates an IS NOT NULL expression with the given arguments.
 *
 * @param string $x Field in string format to be restricted by IS NOT NULL.
 *
 * @return string
 */
public function isNotNull($x)
{
    return $x . ' IS NOT NULL';
}

So there is no good reason for using anything other then just plain DQL here.