Comment 33 for bug 278112

I may have found some problems with the proposed solution (i.e. unchecking "Undirect fullscreen windows") when you have multiple users. Could someone please try these steps and confirm?

Firstly, "uncheck" "Undirect fullscreen windows" in CCSM so that the screensaver works.
Also, make sure that the User Switcher applet preferences are on its default setting, with "Lock screen after switching users" "checked".
Set the screensaver preferences to its default setting with "Lock screen when screensaver is active" "unchecked". (I tried with it checked as well, and don't think that it makes a difference.)
Now create a new user if you don't already have another.
Without logging out of the first session, switch to another user.
In the new session, "uncheck" "Undirect fullscreen windows" in CCSM so that the screensaver works for the new user.
Switch between the two user sessions using the User Switcher applet on the panel.
(I get a white screen instead of the password prompt. If I blindly type the password it will switch sessions and restore the display to normal.)

Assuming you still have two user sessions logged in, "Suspend" the computer (standby) rather than shutting down normally.
Resume the computer from its standby state.
(I get a totally black screen except for the mouse cursor, but there is display corruption in a small square around the cursor. Again, blindly typing the password will usually restore the session.)

I've found 2 workarounds to the white-screen problem.
The first is to click on "System, Log out, Switch user" rather than using the fast user switcher. This seems to always take you to the GDM login screen, rather than the usual locked-screen window, and prevents the white screen.
The second is to "uncheck" the "Lock screen after switching users" in the fast user switcher preferences for both users. This again avoids the locked-screen password prompt, which seems to be causing the white screen problem when using the fast user switcher.

I have no idea how to fix the black-screen on resume issue, except by leaving "Undirect fullscreen windows" "checked". Once again, this may be an issue with the locked-screen password prompt itself, or how compiz uses it.

I've also believe that setting the screensaver preferences to "Lock screen when screensaver is active" is more of workaround than a real solution to the screensaver not starting. With this option checked the screensaver should start immediately after the pre-set time, and only show the password prompt when there"s activity again (move the mouse/keyboard/whatever). With compiz on, it shows the prompt, and after it times out (login period expires), only then does the screensaver start. Turn off compiz and you'll see what I mean. The screen-lock seems to prevent compiz from blocking the screensaver.

Disabling compiz for all users also fixes all these issues, and then everything works as advertised.

-Nvidia 8600GT with version 177 restricted drivers