How to redirect user after form submit in overlay mode?
This seems to be the working solution:

Inside your form:

$form['somebutton']['#submit'] = array('your_custom_callback');


Add a custom callback

function your_custom_callback($form, &amp;$form_state) {
  //redirect users to Drupal.org
  $url = "http://drupal.org";

  if (module_exists('overlay') &amp;&amp; overlay_get_mode() == 'child') {
    unset($_GET['destination']);
    overlay_close_dialog($url, array('external' =&gt; TRUE));
    $form_state['redirect'] = FALSE;
  } else {
    $form_state['redirect'] = $url;
  }
}