Comment 9 for bug 994612

I posted some details on this in a comment at http://askubuntu.com/questions/59577/replace-xscreensaver-with-gnome-screensaver-xubuntu before finding this.

The article details on how to deal with it in 11.10 and how the solution doesn't work (because gnome-screensaver-preferences is missing) in 12.04.

It would seem that a fix for 12.04, which is an LTS after all, would be easy and worth doing. I'd do it (port the gnome-screensaver-preferences script) myself but I'd have to learn enough python first; I wish I had time for that.

My comment gets you to where gnome-screensaver is at least running and locking the screen off the user submenu (top right).
You'll just have a blank screen with the date and time in small chars at the top; press any key to get the login dialogue.

There is a citation link to this page in the comment.