Comment 2 for bug 706251

Not sure of this works for everyone, but I managed to stop all these things from happening and get the screensaver working in Kubuntu 11.10 opening Home/.kde/share/config/kscreensaverrc
In this small file you should find
[ScreenSaver]
PlasamEnabled=False
Saver=<Your Chsoen Screensaver>
Timeout=60 - this is in seconds, I've got mine at 60 for 1 minute for testing.

Just change the line that reads PlasmaEnables=False to PlasmaEnabled=True
(The capitalization is important) save the file and all should be well.