Now the problem with Ctrl-Alt-Backspace is that you can, in fact, lose data. But as a last resort, having this behavior back can really be helpful. And fortunately, it is possible to get the old standby behavior back. This tutorial will show you how. More here

In the most recent release of Ubuntu (9.04) the developers decided (after a very lengthy discussion) that the old tried-and-true Ctrl-Al-Backspace key combination would no longer kill the X server. Their justification for this (in a nutshell) was that this combination could be accidentally hit causing the user to lose data.