Services (top level page)
- Service group 1 (second lvl)
-- Service group 1.1 (third lvl)
--- Service 1.1.1 (4th lvl)
--- Service 1.1.2 (4th lvl)
--- Service 1.1.3 (4th lvl)
-- Service group 1.2 (third lvl)
--- Service 1.2.1 (4th lvl)
--- Service 1.2.2 (4th lvl)
-- Service 1.2 (third lvl)
-- Service 1.3 (third lvl)
- Service group 2 (second lvl)
- Service group 3 (second lvl)
- Service group 4 (second lvl)

So the bad things here:
1. Layouts: top level service page, second level service group page, third level service group page and a separate service (no matter what level) page had its own layout
2. Structure: service groups have minor service groups. Still, both, minor and major, groups can have their own services.

The thing here is that _I could have_ used is_tree() if I only had service groups that have services, but if the structure is complex, it is useless and produces a lot of code and if. Hence I went the simple way: I made different page templates and let the user set them. It is much more dynamic that way and requires no coding at all - copy the structure, reshape it and produce another layout.

I don't vote :)

Your passionate advocacy of is_tree sounded just like what i needed to fix my submenu problem... but I can't get it to work *&%$#(@! Can you or someone help me? Newbee Wordpress guy.

I have a double scissor-type drop menu, that wouldn't work with wp-list pages so I'm hardcodeing my navigation. You can see it here at http://wereorganized.biz/.

I pasted the is_tree function(exacty as from your post) into my functions.php, then added this to the list item that contained the nested unordered list of my submenu:

<li<?php if (is_tree('garage-cabinets')) { echo " class=\"current\""; } ?>><b>GARAGE CABINETS</b>

I was expecting the "current" class to be added to any submenu li of garage cabinets, what I got was this error:
Fatal error: Call to undefined function is_tree() in /home/content/77/5846477/html/wp-content/themes/modern-style/header.php on line 57

Can you give me a clue as to what I'm doing wrong?

The ability to say if (is_tree('services')) {//do whatever} else {//do something else} means that ANY NEW pages you add under services (but NOT sub-sub-pages) will trigger whatever instructions you provide when a sub-page of services is displayed.

Does that help? Rather than having to declared all existing pages and give the instructions and hope that no new pages are added... you can account for any new pages via the is_tree() function.

Why should it have to be added to the functions.php file when it could simply BE in the core functions and ready to roll?

My two. HTH

Could you elaborate on why you'd want to display subpages on the parent page?

I'd like to see something that would allow me to nix the drop-down menu in favour of having the sub-pages appear directly underneath the content of the page. So, when you'd click on the page name, scrolling down you'd first see the page content and then as you kept scrolling, you'd see the content of the sub-pages, organized just the way entry follows entry on the blog side of things.

You can read about this here:
http://codex.wordpress.org/Conditional_Tags

Scroll down to Snippet #4.

Here's the function, but you have to add it to use it, since it ISN'T built-in.

function is_tree($pid) {      // $pid = The ID of the page we're looking for pages underneath
	global $post;         // load details about this page
	if(is_page()&&($post->post_parent==$pid||is_page($pid)))
               return true;   // we're at the page or at a sub page
	else
               return false;  // we're elsewhere
};