Ever looked at the about:Tabs page in IE8, pressed the right mousebutton and clicked on View Source? Browsing through the HTML and Javascript you find references to window.external all over the place. And there is also the "secret" script include of "NewTabPageScripts.js".

You might have tried the Developer Tools (press F12) to find the secret script's source code, but had no luck with that. Also typing about:NewTabPageScripts.js in the addressbar didn't made you jump up from your chair, screaming some euphoric words to be remembered by your offspring...

What you could do, is type in the following line in your addressbar:
  javascript:alert(window.open("NewTabPageScripts.js", "secret"));

Be sure to (temporarily) allow pop-ups!

A new tab (or window, depending on your tab-settings) opens, showing you the sourcecode of the secret javascript.
How is this possible? The Javascript line which creates the new window, is evaluated within the same context as the about:Tabs page. From within that context, IE is able to load the .js into the script element. Therefore it must also be able to load it in a new window from that same context.

When you look at the address bar of the newly opened tab/window, you'll find the following url:
  res://ieframe.dll/NewTabPageScripts.js

The res://ieframe.dll/ points to the "HTML" resource folder in ieframe.dll (located in your system32 folder). It's a resource dll with a lot (maybe all) of the about: pages. Use a resource viewer to look at all the bitmaps, HTMLs, scripts, GIFs, PNGs, AVIs, icons, dialogs, menus, strings and binaries.
If you have Visual Studio, just drop the ieframe.dll on the main window. A new page opens showing you all the resources.

The New Tab page itself, is also located in the resource dll:
  res://ieframe.dll/TabsWelcome.htm

Just showing it in plain sight...