Version:           4.2.0 (using Devel)
Compiler:          gcc 4.3 
OS:                Linux
Installed from:    Compiled sources

Using the following code in a function does not cancel the screen saver in KDE 4.2.0. (To avoid doubt, the screen saver is the ordinary sort, not a powersave suspend.)

KNotificationRestrictions n(KNotificationRestrictions::ScreenSaver);

The D-Bus call in KNotificationRestrictions::Private::startScreenSaverPrevention() returns reply.isValid() == true, but the screen saver stays active.