I couldn’t find how to enable monitor power save mode in Mandriva 2009.0 KDE — the screen saver just kept going and going without ever turning the monitor off.
It’s really quite easy — it’s just that the setting has moved from screensaver options to monitor options:
Just go to “Configure Your Desktop” -> Display -> Power Control
(Thanks to a forum post on 0×61.com for leading me toward the right idea!)