Sharing the Permissions Table

I would highly recommend against it. This will basically lock you into needing to have the same modules running on all sites indefinitely. A better option would be to share those few tables you mentioned and create a Feature via http://drupal.org/project/features that can be used to keep permissions in sync. There's a bit more leg-work in checking boxes but this will allow you to have additional modules activated in the future. Actually a lot of stuff can be standardized via Features now so you really don't need to share role either (though there isn't much harm in doing it).

I'd still make sure that the databases you setup have a users, sessions, role, and authmap table included in them. This is because while query code is supposed to be wrapped in {} for tables (which tells drupal to snag the referenced table in-case it's different like in shared tables) not everyone always remembers to do that. Having these tables still exist (just not be used) is a good fail-safe. Also what I like to do is have the users table per site only have my user 1 account in it. This way if you need to "lock a site down" but still work on it and access the info you can just edit the settings.php file to point to the normal users table instead of the shared one.

Interesting. had no idea features could capture perms. I'll see what I can do. I'm trying to eliminate as much redundancy as possible in my setup.

I have about 12 sites in the multisite environment. It's for a hospital, with similar departments/divisions to a university. Basically using multisite to divide up larger potions of the site so it's not too overwhelming for users of the cms.