Comment 22 for bug 528094

vamped wrote in post #21:
believe that <control><alt><l> is Ubuntu's default lock-screen key-combination. It works for me (activates xscreensaver lock) and I have not added it as a custom command. Moreover, I'm using the default visual-effects.

Yes, that is the default lock-screen combo. It didn't work in Jaunty, I don't think it works in Karmic without the Compiz command, but I haven't used Lucid. Good news that it works for you!

Now the question is: can you lock screen from the session menu using xscreensaver in Lucid?

I haven't upgraded yet but probably will soon (currently Karmic).