Use “org.freedesktop.ScreenSaver”, that should work for both KDE and GNOME, and probably other DEs as well.
qdbus org.freedesktop.ScreenSaver /ScreenSaver SimulateUserActivity
Or see here for how to temporary disable/enable it:
http://people.freedesktop.org/~hadess/idle-inhibition-spec/re01.html
I.e. something like
cookie=`qdbus org.freedesktop.ScreenSaver /ScreenSaver Inhibit someId reason`
(where someId should uniquely identify your application, and reason is a string that describes the reason)
Afterwards you can enable it again with:
qdbus org.freedesktop.ScreenSaver /ScreenSaver UnInhibit $cookie