Hi,
The default behavior for someone trying to reboot or even shutdown a system when someone else is logged in is to just to log them out kick them back to the login screen, without even explaining. For instance, I set up two separate accounts on my then girlfriend's machine. She was an admin, I was not. Whenever I would use the machine, I would log in, sometimes I forgot to log out. Then, as a novice user, she would not be able to shutdown the machine. I wound up having to teach her how to open a terminal and issue 'sudo poweroff' in order for her to override it.
We've received a request to change this for an OEM, and on their pre-install images we have a policy that allows you to power off even if someone else is logged in. We used the instructions here: http://ubuntuforums.org/showpost.php?p=11901595&postcount=7
I'd like to get this into Ubuntu proper, seems like policykit-desktop-privileges is the perfect place for it.
Just tested, this branch works for me.