http://hal.freedesktop.org/docs/polkit/
Basically polkit let's you allow a user hussam for example to change the system date/time only and a user samer to only be able to update packages without entering a password. If something wrong happens and some exploit is found, it'll be dbus's fault.
I found this screenshot.
http://img.skitch.com/20090709-giesda4e32erc7y2gt29dpjab4.png
DBUS API Docs.
http://dbus.freedesktop.org/doc/api/html/index.html dbus allows inter-process communication.
For example, your IM program can use dbus and your media player can use dbus then your IM can set your status to "Now listening to: SongX by ArtistY"
For distributions that have a "GUI frontend" for every administrative task imaginable, you can fine-grain permissions but it'll be like "roles" instead of user permissions.