If you have the problem that your ctrl-alt-backspace combination does not work and you have to restart the x-server see this blog entry here for the solution or simply go to System –> Preferences –> Keyboard –> Tab „Layouts“ –> Button „Options“ –> open „Key sequence to kill the X-Server“ and enable the checkbox. Done.
joergweis
Software development since 1994 now, specialized on large scale search engines in an enterprise environment. I am a dedicated ubuntu evangelist, because this distribution currently drives out the most of the desktop in a productive manner. After some successfull years leading an operations department of a big yellow pages search engine provider in Austria I finally moved on to one of the biggest IT companies in the world where I am currently responsible for strategic outsourcing project solutioning for our clients.
Jörg@Home's WeBlog 
Eine Antwort zu “Enable Restart X server (Ctrl-Alt-BackSpace) in Ubuntu 11.04 Natty Narwhal”
Das Problem gibt es ja schon seit Ubuntu 9.04.
Hier ein Blogpost dazu, welche Möglichkeit es noch gibt den X-Server zu starten:
http://www.ubuntuverse.at/2009/06/09/x-server-restarten-unter-ubuntu-904/
Tastenkombination ALT+DRUCK+K