As stated in the Ubuntu 9.04 release notes, Ctrl-Alt-Backspace key combination to restart Xorg is disabled.
Ctrl-Alt-Backspace disabled by default in Xorg
The Ctrl-Alt-Backspace key combination to force a restart of X is now disabled by default, to eliminate the problem of accidentally triggering the key combination. Users who do want this function can enable it in their xorg.conf, or by running the command
Ctrl+Alt+Backspace Again Ubuntu Linux 9.10 Karmic Koala