I'm not seeing this so I assume it's not a feature, so this is a feature request; this might be something major enough to plan out for a 3.0 version really....
I'm considering hosting multiple blogs on one install of WordPress. This means many bloggers, many blogs, individually themed, but one code base. Upgrading, thus, becomes easier due to centralized management.
Consider first the example of an open source software dev team, let's call it BLONG. There's maybe 40 or 50 core developers; today, this is 40 or 50 WordPress installations, individually managed and controlled, and individually upgraded (you hope).
Instead, consider that BLONG's core devs set up a WordPress, with core administrators. Every core dev gets their own blog on this code base. This isolation gives:
- Their own blog title
- Their own blog description
- Their own plug-in set
- Their own themes
Blog title and description are easy. Active theme and plug-ins are easy. Implementing isolated theme or plug-in addition is hard because each user needs his themes to stay under his control, so he can update them to new versions yet he can't trounce all over other users' themes or force them to update/downdate their plug-ins.
Other considerations include schema modifications and such for the database, but... like I said, this is a big feature request. It's a possible plan for the next major, not a minor enhancement you'll put into 2.3.