On my site I wanted to have tab linked to Gallery installed in different place. It appeared very easy to achieve ;).
In my theme folder I had to change header.php file.
It looked like this:

<ul>
<li <?php if(is_home()){echo ‘class="current_page_item"’;}?>><a href="<?php bloginfo(’siteurl’); ?>/" title="Home">Home</a></li>
<?php wp_list_pages
(’title_li=&depth=1&’.$page_sort.’&’.$pages_to_exclude)?>
</ul>

And now it looks like that:

<ul>
<li <?php if(is_home()){echo ‘class="current_page_item"’;}?>><a href="<?php bloginfo(’siteurl’); ?>/" title="Home">Home</a></li>
<?php wp_list_pages
(’title_li=&depth=1&’.$page_sort.’&’.$pages_to_exclude)?>
<li><a href="http://www.nietoperzka.com/Gallery2/gallery2/main.php" target="_blank">Gallery</a></li>
</ul>

Iiiiiiha ;).