Thanks for that
I've been using a shortcut to xscreensaver-command -lock, but I hadn't thought of the fact that someone could simply Ctrl+Alt+F1 to get back to my main console. I just tried out lock -nv, which works great. Even with X going, as long as you do it from a console instead of an xterm, it seems to be bulletproof.
This is a timely post for me, because I'm just getting ready to head out the door to the Synergex SPC in Sacramento -- not that I intend to leave my laptop lying around.