How do you use the Drupal 7 overlay on the node/edit screens for your popups when it's already being used? I'm writing a module that adds a button to node/edit forms. This button needs to popup a DIV/IFRAME in a light box. I notice that Drupal 7 uses the new Overlay module for administrative and node/edit pages.

So my question is as my button is on a node/edit page I need to overlay the overlay. Is there a way to access the overlay and change or z-index or do I just write my own overlay code to create a light box effect (overlaying the node/edit overlay)?
It is possible to change the z-index used by the overlay, but I don't think it's necessary. You just use for your overlay a z-index that is higher than the one used by the Overlay module.

In the case you need to know when the overlay is output, or how it is output, you should check the code executed by overlay_page_alter(), which is the implementation of hook_page_alter() made by overlay.module.

function overlay_page_alter(&amp;$page) {
  // If we are limiting rendering to a subset of page regions, deny access to
  // all other regions so that they will not be processed.
  if ($regions_to_render = overlay_get_regions_to_render()) {
    $skipped_regions = array_diff(element_children($page), $regions_to_render);
    foreach ($skipped_regions as $skipped_region) {
      $page[$skipped_region]['#access'] = FALSE;
    }
  }

  $mode = overlay_get_mode();
  if ($mode == 'child') {
    // Add the overlay wrapper before the html wrapper.
    array_unshift($page['#theme_wrappers'], 'overlay');
  }
  elseif ($mode == 'parent' &amp;&amp; ($message = overlay_disable_message())) {
    $page['page_top']['disable_overlay'] = $message;
  }
}