If you want to quickly restart X Windows on Ubuntu or any version of linux, all you have to do is hit this key combination quickly or use the command line option
You can use the following command from your terminal
sudo /etc/init.d/gdm restart
Note: Make sure that you save your work before doing this
Thanx, but on Ubuntu 9.04 it does restarts the PC, not only the session.
Any other option on how to restart the session, or just restart the desktop.