Need

[eluser]Michael Wales[/eluser]
This isn't a complete example, just the parts that matter

Code:

$rules['email'] = 'required|valid_email|callback__check_email';
$this->validation->set_rules($rules);

function _check_email($email) {
  $query = $this->db->get_where('users', array('email'=>$email), 1, 0);
  if ($query->num_rows() != 0) {
    $this->validation->set_message('_check_email', 'That email address has already been registered.');
    return FALSE;
  }
  return TRUE;
}

Should be enough to get you on your way.