CTRL-ALT-Backspace has been depricated for about 2 years now because it is an ungraceful way of doing things.
As this is not a funktion of the desktop environment, LXDE in your case, but a function of the x-server, you can reenable it in /etc/xinit/xinitrc by adding:setxkbmap -option terminate:ctrl_alt_bksp
greetz
devil