This is an incredibly infuriating problem.
Have a Mac Mini running 10.5.8 using a wireless aluminum 2007 keyboard and this problem suddenly kicked in.
I'm guessing it's as a result of an update but hard to pinpoint when exactly this started happening.
Tried enabling/disabling the 'Use all F1,F2, etc' option under the keyboard and mouse settings but no change.
Would love to be able to roll that last few updates back or get under the hood of the keyboard preferences as repairing the keyboard does nothing.
Have been trawling the web for any solution to this but no joy at all.
Disappointing, have learned the lesson of not upgrading when everything works perfect (if it ain't broke don't fix it and all that)