I am having exactly this issue described on bug #141623 report:
It seems like ever since a system update earlier this week (one that updated a number of things, but acpi packages stuck out), my Fn keys no longer control the brightness.
Up until that update, Fn+F6 and Fn+F7 would decrease and increase the brightness, respectively. Now it can be controlled with the Power Manager, but not through the Fn keys. Again, this worked correctly until a recent update.
This bug has been closed but after I have installed Kde 4.1 I am experiencing it.. anyone knows how to get these keys back?
Another info: I can see the OSD level everytime I plug or unplug the power chord. Nevertheless.. fn keys just don do nothing anymore