This is what we've got at the moment: http://cgit.freedesktop.org/xorg/xserver/tree/randr/rrproperty.c#n26
RRDeliverEvent (ScreenPtr pScreen, xEvent *event, CARD32 mask)
As this function does nothing, clients do not get notified when the OutputProperties change. This means that clients like gnome-power-manager that want to be notified if anything else has set the brightness don't get any events and the two programs get out of sync.
Fixed in 9187f6ad9ec7ba9569a93d92561aac17eaa83491 and following.
Legend, thanks. Looks good to me.