As an update for XBMCbuntu, you can run the following command to start/stop XBMC:
sudo service lightdm [start|stop|restart]
I always forget the command for restarting XBMC Live from the shell, so I figured I would post it to my blog. It's a really simple, one-line command:
sudo /etc/init.d/xbmc-live restart
Then enter the password for the root account (typically 'xbmc').
Just to be clear, the benefit of running this command instead of a full reboot (sudo reboot) is the above command only restarts the XBMC Live application. This means if you have any other services or apps running (like SABnzbd+), you can keep those apps running.
The only reason I've ever really needed the xbmc-live restart command is when something gets locked up on XBMC... and that usually only happens when I'm trying out new skins or plugins.
I should also note that on occasion this command will not restart XBMC live. The alternative is to stop, then start XBMC again using the following commands:
sudo /etc/init.d/xbmc-live stop
sudo /etc/init.d/xbmc-live start