- xscreensaver+gdmflexiserver work well, it automatically locks, but clicking on "New Login" presented by xscreensaver it fails with the message about gdmflexiserver no longer being supported. This makes letting other users log in practically impossible.
- gnome-screensaver never automatically locks. I can lock it with Win+L, and everything is OK. But if I forget to lock it when I leave my desk, the screensaver does not lock, no matter what options I have in gnome-screensaver-preferences or gnome-power-manager.
The xscreensaver+gdmflexiserver is a problem, but it's not a security problem. I am the primary user of this machine, so using this option would work well for me 95% of the time. It's when others want to use my computer that this becomes an issue. I don't want them to reboot the machine while I'm not here, but I want them to be able to launch their own GNOME session without impinging on my normal session.
I only need a solution to one problem; if I solve one of the above problems the other problem will become immaterial. If another display manger (e.g. Slim) would work better for this purpose, by all means recommend it.