Ctrl+Alt+Backspace (the shortcut which was used to restart the X server) has to be enabled in a different way starting in Ubuntu 9.10 (Karmic Koala).
- Get to the System->Preferences->Keyboard menu.
- Select the “Layouts” tab and click on the “Layout Options” button.
- Then select “Key sequence to kill the X server” and enable “Control + Alt + Backspace”.
Click the link below for the full article and how to do it in KDE.
Source: Ubuntu Geek.