Comment 23 for bug 528094

[1] indicates that indicator-session calls gnome screen saver directly. It should do whatever ctrl+alt+l does so that it would work with xscreensaver.

Why I'm interested? Switch user does not lock current session so I can switch between users without entering a password as long as xscreensaver timeout for lock is not hit.

Why I'm using xscreensaver? Because there's a bug [2] in gnomescreensaver that forces me to use xscreensaver for now, fix has been done, but is not yet available in standard repositories.

[1] https://bugs.launchpad.net/ubuntu/+source/indicator-session/+bug/436724
[2] https://bugs.launchpad.net/ubuntu/+source/xorg-server/+bug/546578