I can dim my laptop's display just fine in KDE 4.5 but can I dim my desktop's display? I would like it darker at night and brighter during the day, but I can't adjust it under Power Management. It is attached via VGA if that makes a difference.

As far as I am aware this isn't possible, as laptop monitors use a special interface with the system, involving more than just the graphics card, which allows them to control the brightness among other things.

After thinking about this for a while I have come to the conclusion that what I am really looking for is not a power management feature but a KWIN effect that would dim the entire interface. The effect would be similar to the "Dim Screen for Administrator Mode" effect already in KWIN. The effect would run on a timer which would be configured by the user. It could dim the display at night time so users working late into the night don't get their eyeballs blasted with too much light.

Is this possible?

A KWin effect could possibly implement this, not sure if the KWin developers would implement it though.