I can't imagine how this would allow for that functionality (Although I agree it could be cool). With current Drupal 7/MySQL setup each site has absolutely no knowledge of the other sites. I worked about 40 Drupal sites (most on the same domain) at my last job. Many had very similar structure, modules, themes, etc, just a different content source and it seemed to make the most sense to the devs before me to create separate sites (I don't necessarily disagree).

To make updates to core, modules and the like, I wrote a couple scripts and a config with a list of the siteroots. It would cd to each directory then run the drush (the CLI client/module) commands we wanted. Before that, we had each engineer grab a couple sites and run the commands after we deployed. Less than ideal.

Seems silly for common modules or core updates.

First, the sites would have to have some kind of knowledge of each other.

If you're looking for structural sharing, you can export views individually, but the majority of the core settings can be exported as a module with the features module. Users can be shared through the use of shared tables in the settings.php file.

If you're looking for content movement/sharing, perhaps Organic Groups is worth a look.

(edit: forgot the letter y)

drupal already do it starting with version 5.x (current stable one - 7.x)