Steps to reproduce in Drupal 6.9:

  1. Try to authenticate with an OpenID (assuming there is no user with this id)
  2. Fill some illegal data, so that autoregistration fails
  3. The registration form is displayed, but no data is prefilled

As far as my research goes the problem is in openid.module function openid_form_alter (line 117). The code is:

  elseif ($form_id == 'user_register' && isset($_SESSION['openid'])) {
    // We were unable to auto-register a new user. Prefill the registration
    // form with the values we have.
    $form['name']['#default_value'] = $_SESSION['openid']['values']['name'];
    $form['mail']['#default_value'] = $_SESSION['openid']['values']['mail'];
    // If user_email_verification is off, hide the password field and just fill
    // with random password to avoid confusion.
    if (!variable_get('user_email_verification', TRUE)) {
      $form['pass']['#type'] = 'hidden';
      $form['pass']['#value'] = user_password();
    }
    $form['auth_openid'] = array('#type' => 'hidden', '#value' => $_SESSION['openid']['values']['auth_openid']);
  }

The values for the form should be set using $form['account']['name'] for instance, and not $form['name'].