I use the well-known screen saver Ciao to automaticly take my computer to the login window without logging out the users (via fast user switching) after a designated time period. I do this not so much because of the supposed security concerns, but because in my household, everyone knows each other's password (and quickly discovers it if I change it), and this way it's just as easy to login to one's own account as it is to use the one that was last being used.
Anyway, the problems with this set up are:
- There isn't any easy way to set up a default behavoir for Ciao.
- Anyone can change their screensaver, and hence ruin the whole thing.
So here is what you need to do. Changing the default behavoir of the screen saver is pretty straightforward. Make sure Ciao is installed for all users, and then modify /System -> Library -> Frameworks -> ScreenSaver.framework -> Versions -> A -> Resources -> EngineDefaults.plist to match the following:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist SYSTEM "file://localhost/System/Library/DTDs/PropertyList.dtd">
<plist version="0.9">
<dict>
<key>askForPassword</key>
<integer>0</integer>
<key>hotCorners</key>
<array>
<array>
<string>0</string>
<string>0</string>
<string>1</string>
<string>1</string>
</array>
</array>
<key>idleTime</key>
<integer>300</integer>
<key>moduleName</key>
<string>Ciao</string>
<key>modulePath</key>
<string>/Library/Screen Savers/Ciao.saver</string>
</dict>
</plist>
This sets Ciao as the default saver, sets the timer interval to five minutes (300 seconds), makes both bottom corners activate the saver, and turns off ask for password (since it would be redudant to do so). Now we need to set these options in stone.
Navigate to /System -> Library -> PreferencePanes -> DesktopScreenEffectsPref.prefPane -> Contents -> Resources in the Tterminal. Once there, type sudo chmod 750 ScreenEffects.prefPane/. That gives the owner read, write, and execute permissions, and the group read and execute permissions. Make sure to change the owner to your administrator account. The permissions should now read:
drwxr-x--- 3 admin_user wheel 102 Mar 20 2005 ScreenEffects.prefPane
Now you are set to go. You should be able to open and change things in the Desktop & Screen Saver pane as normal in the admin account, but trying to do so in any other account will go nowhere. It won't even crash the System Prefs app, it will just sit there until you click something else or quit. Nice.
One caveat: users cannot change their Desktop either due to the combination (in 10.4) of the Desktop and Screen Saver pref pane. This may or may not be a good thing, depending on your set-up. For me, it works. Perhaps you can reclaim this functionality by installing Panther's Desktop pref pane (in that case, you could simply remove the Desktop & Screen Saver prefpane, and modify things via the plist). Anyone?
[robg adds: This hint changes the ownership and permissions of a system-level file. I expect that if you were to run Repair Permissions in Disk Utility, this "problems" would be "fixed." It may also get reset after system updates, so you'd have to keep an eye on it.]

