drupal_get_destination() has some internal code that points at the correct place to getthe current internal path. To translate that path into an absolute URL, the url() function should do the trick. If the 'absolute' option is passed in it will generate the full URL, not just the internal path. It will also swap in any path aliases for the current path as well.
$path = isset($_GET['q']) ? $_GET['q'] : '<front>';
$link = url($path, array('absolute' => TRUE));