[development] Take Over Page Output from Module

[Adam B. Ross]

The Print module, Book module's export functionality, the s5 module,
they all have this trick of taking over the entire page output for
their own theme function.

I am building a module that is headed toward a more generic mechanism
for funneling content through a pluggable template, but despite
pouring over the code I can't see the magical place where all other
Drupal markup is dropped in favor of the HTML document described in my
own template file.

What's the trick?

I have a menu callback along the lines of "present/%node" and it leads
to an export function that returns my theme function, which is a
template outputting a complete HTML document.