Possibility of "Show All" link?

To add a "Show all" link in the pager, override the theme('pager') function and add the following right before the return statement:

    $items[] = array(
      'class' => 'pager-all',
      'data' => l(t('Show all'), $_GET['q'], array('query' => array('page' => 'full'))),
    );

I do not plan to ship an overridden theme_pager in paging. Let Drupal maintain that part. :)

As I tested, altering the theme works... but only for showing the full content. Switching back from full to paged content does not work (seems pager is not present at all in this case).
I altered the function paging_link, adding following at the end (before return statement):

  if ($type == 'node' && !$teaser && $node->page_count > 1) {
    // Paging is applicable
    $page = isset($_GET['page']) ? $_GET['page'] : '';
    if (arg(2) != 'print' && arg(2) != 'full' && $page != 'full') {
      // Paging is active now
      // Add "Show full page" link
      $links['paging_show_full'] = array(
        'title' => t('Show full page'),
        'href' => drupal_get_path_alias("node/$node->nid"),
        'attributes' => array('title' => t('Show whole content in single page (no paging).'), 'class' => 'read-more-paging'),
        'query' => 'page=full'
      );
    }
    else {
      // There is full display active for content where paging exists
      // Add "Show paged" link
      $links['paging_show_paged'] = array(
        'title' => t('Show paged'),
        'href' => drupal_get_path_alias("node/$node->nid"),
        'attributes' => array('title' => t('Show content divided to pages.'), 'class' => 'read-more-paging')
      );
    }
  }

The "Show full page"/"Show paged" are in the links section, so they are always displayed whenever the paging should apply for the content.
This fits my needs better than theming the pager.
If there would be additional option for this, it could be even configurable (just as for "Read more" link) so it would not disturb those who do not want to use it... and theming of the pager would still be possible for those who prefer that way...

Is there a chance to have this feature built in module?

I think we can make it so that pager is still available even in full view.

Does this also work in D5??