Joao: you can configure XScreenSaver to show a “Switch User” button in it’s password dialog (which calls gdmflexiserver when run). This lets you start a new X session after the screen has locked. This feature is turned on in Ubuntu if you want to try it out.
Of course, this is not a full solution, since it doesn’t help you switch to an existing session (you’d need to guess the correct Ctrl+Alt+Fn combo). There is code in gnome-screensaver to support this though, giving you a list of sessions you can switch to.
Although it doesn’t list sessions that are currently logged in, if you try to log in again with GDM, it brings up a prompt asking you if you want to start a new session or return to your other one. So while perhaps not as convenient as allowing you to see which users are logged in, you don’t have to just guess the correct Fn key to press.
For starting a new session from an existing one, you can always just make a launcher for gdmflexiserver. Fedora Core 4 has a precreated one in Applications->System Tools->New Login in the GNOME menus; I don’t know about Ubuntu.
Craig: I’m aware that there is a menu item to run gdmflexiserver (jvic mentioned it too in the post this one replies to).
That doesn’t help you if you’re sitting at a workstation that someone else has logged into and locked the screen on. This is where the “switch users” button comes in handy.
I’m fairly sure only gnome-screensaver has this switch user button, not XScreensaver. Or did Ubuntu patch XScreensaver?
anonymous: the feature has existed in xscreensaver since about March. It may not be turned on by default though.
How can one activate this feature?
What does one need to configure?