Using xubuntu 13.04 The screen is not dimmed on inactivity even though the xfce4-power-manager-settings for the monitor is set up in a way that if the computer is plugged in, the monitor shall dim the screen brightness to 60 % after 30 seconds. It is the same for plugged-in operation or on battery operation. This has been working with gnome before, so I know that my laptop is generally able to dim the screen. Standby for the monitor does work.
I also use Xubuntu 13.04 (64-bit) and I have the same problem. I think this might be related to this: http://xfce.10915.n7.nabble.com/PATCH-Fix-reduce-screen-brightness-feature-for-64bit-builds-td41960.html
Please retry with up-to-date version - at the moment this would be 1.4.1 - and reopen if the problem still exists.