No matter if I disable or enable the "Monitor Power Management Control" checkbox, my monitor never turns off even after the "Switch off display when computer is inactive for" time has passed. I gather that there is some sort of known interaction with xscreensaver (see eg. http://www.planetwatt.com/forums/topic/77/xfce-power-manager-does-not-swit), but I couldn't find a bugzilla issue for this yet.
This is a known issue but one the power-manager can do nothing about. XScreenSaver consciously uses its own DPMS settings and overrides the ones by X11 (which is what the power-manager is using), hence the settings in the power-manager don't apply. Hence you have to change the settings in XScreenSaver's settings dialog.