When I try to shutdown, at first nothing happens. This is true whether I click shut down (or hibernate, reboot, suspend) on the username dropdown menu of the Panel, or use Fn+F4 (it's a laptop). On the 2nd attempt I get an error message: Failed to shut down. Session manager must be in idle state when requesting a shutdown. For older versions there have been suggestions of setting the hostname so that it doesn't have any spaces in it - mine doesn't anyway. This has only been happening after upgrading to 12.04 and installing xfce (I didn't hang about for long enough in between to test on the default 12.04 installation). This isn't an xubuntu install - I went through the upgrade path from 11.04 then installed xfce on top. I'll test shutting down from the terminal - for obvious reasons I don't want to do it while writing this post! sudo shutdown -r now works fine.
(edited - more info re terminal shutdown, +line breaks for clarity)