xev correctly recognizes the buttons as XF86MonBrightnessDown and XF86MonBrightnessUp.
In System Settings -> Keyboard & Mouse the display brightness keys are correctly recognized as brightness up and down when clicking "Custom key" for a random action and pressing those buttons.
This means that X11 and Qt do know that the buttons are pressed.
xev correctly recognizes the buttons as XF86MonBrightne ssDown and XF86MonBrightne ssUp.
In System Settings -> Keyboard & Mouse the display brightness keys are correctly recognized as brightness up and down when clicking "Custom key" for a random action and pressing those buttons.
This means that X11 and Qt do know that the buttons are pressed.
Probably the bug is in KDE, then?
Btw, this is a regression from Kubuntu 9.10.