I have similar concerns. These and some other examples of pages do not use theme('page'), so it must be possible for a menu callback to output without that wrapper. This can probably be managed with special-casing "return NULL;" to not pipe output through theme('page'), as suggested by Karoly.
right - problem solved.
To make this change, you also have to police the entire code base for the many uses of drupal_set_title(), menu_set_location(), etc. At the moment all menu callbacks can assume that they are in charge of the entire page, and many use the above functions and others which would be detrimental to using the output in other contexts.
again, not a big deal IMO. The callign code can always set the title as it wishes. It doesn't matter if the original page function had a title/breadcrumb or not. This is minor.
One possible benefit of such a huge refactoring would be the ability to grab smaller pieces of a page outside of the page context somehow, for use in the Ajax stuff I'm playing with. (If you're interested, play with the table sorting widgets at http://jonbob.drupaldevs.org/api/head/group)
Huge benefit! Love the demo. The code is squeeky clean. That drupal.js file is stunning in its simplicity and usefulness. PS: lets stay on topic here. Karoly's suggestion has nothing to do with changes to themes.