The relevant code:

// app/views/user/login.ctp
flash('auth');

echo $form->create('User', array('action' => 'login'));
echo $form->input('username', array('label' => 'OpenID:'));
echo $form->input('password', array('type' => 'hidden'));
echo $form->end('Login');

// app/model/user.php
public function find($conditions = null, $fields = array(), $order = null, $recursive = null) {	
    if (is_array($conditions) && isset($conditions['User.username'])) {
        $newconditions = array('User.openid' => $conditions['User.username']);
        return parent::find($newconditions, $fields, $order, $recursive);
    }
    return parent::find($conditions, $fields, $order, $recursive);
}

// app/controllers/users_controller.php
private function handleOpenIDResponse($returnTo) {
    /* the OpenID response */
    $response = $this->Openid->getResponse($returnTo);        
    if ($response->status == Auth_OpenID_CANCEL) {
        $this->setMessage('Verification cancelled');
    } elseif ($response->status == Auth_OpenID_FAILURE) {
        $this->setMessage('OpenID verification failed: '.$response->message);
    } elseif ($response->status == Auth_OpenID_SUCCESS) {
        $openid = $response->identity_url;
        // ugly hack to remove trailing slash, since Auth component strips it and we need to match
        if (substr($openid, -1) == '/')
        {
            $openid = substr($openid, 0, -1);
        }
        $data = array('User' => array('openid' => $openid));

        /* here create the User if it doesn't already exist */
        $user = $this->User->find('first', array('conditions' => array('User.openid' => $openid)));
        /* create user if it doesn't already exist */
        if ($user === false)
        {
            $this->User->create();
        }
        
        /* array of any user details we requested, if the user returned them */
        $sregResponse = Auth_OpenID_SRegResponse::fromSuccessResponse($response);
        $sreg = $sregResponse->contents();
        if ($sreg)
        {
            if(array_key_exists('fullname', $sreg))
            {
                $data['User']['name'] = $sreg['fullname'];
            }
            if(array_key_exists('email', $sreg))
            {
                $data['User']['email'] = $sreg['email'];
            }
        }
        /* save user details */
        $success = $this->User->save($data);
        
        $loggedin = $this->Auth->login($response);
        $this->redirect($this->Auth->redirect());
    }
}

Yes, you are right, this approach no longer works. I tried to find a workaround, but without luck so far…