Whenever I have keyboard issues (or hear about them) I usually use (or recommend) using xev to see what keyboard events are being generated for X. With the function keys, when running xev you should see the following lines reported:
Code:
KeyPress event, serial 32, synthetic NO, window 0x2200001,
root 0x1a6, subw 0x2200002, time 1405600160, (42,48), root:(1281,686),
state 0x10, keycode 67 (keysym 0xffbe, F1), same_screen YES,
XLookupString gives 0 bytes:
XmbLookupString gives 0 bytes:
XFilterEvent returns: False
KeyRelease event, serial 32, synthetic NO, window 0x2200001,
root 0x1a6, subw 0x2200002, time 1405600301, (42,48), root:(1281,686),
state 0x10, keycode 67 (keysym 0xffbe, F1), same_screen YES,
XLookupString gives 0 bytes:
XFilterEvent returns: False
Naturally the keycodes and keysyms will increment 1 for each function key (68: 0xffbf for F2, 69: 0xffc0 for F3, etc).
If these aren't being reported properly then I would suspect your keyboard layout setting in xorg.conf. If they are then something in KDE is probably overriding X.