Comment 5 for bug 805906

I've had similar experiences on a kubuntu install on my main machine - not virtual box.
I select shutdown, the screen greys out a bit - but all the apps are still running and it doesn't get any further; I can still type in a konsole and do a sudo shutdown -h now

Having said that, it's got more broken now, the shutdown option has disappeared from the leave menu - that's bug 853294

Dave