On my HTPC I'd like to ask my Karmic install for my password every time I insert a USB storage device in order to protect it from accidental interference (or theft of files!) by friends/family. I'm happy to have removeable devices automount, but be accessible only after authentication by my user account.

I've Gooooogled lots...
Found this page -> https://help.ubuntu.com/community/Mount/USB
It says:
User Privileges
If your usb device doesn't appear on your desktop, you should check that your user has the correct privileges. Go to System->Administration->User and Groups, choose the user, click on "Properties", then go to the "User Privileges" tab. You should have the "Access external storage devices automatically" option checked.
I've unchecked that option with the expectation that it would do what I want, but it seems to have no effect.

I found this thread (Unable to mount USB, Not Authorized) in which people report the behaviour I want, but (in their case) this was a problem for them. Someone suggested this was a bug. Does the bug work both ways?