I've been having the same problem for some time, it's also persistent with the latest KDE 4 version (KDE 4.2.90 (KDE 4.3 Beta2)) from Kubuntu Backports. All multimedia keys are affected, just as you described it used to work in KDE 3.5.10 (8.04).
I've been having the same problem for some time, it's also persistent with the latest KDE 4 version (KDE 4.2.90 (KDE 4.3 Beta2)) from Kubuntu Backports. All multimedia keys are affected, just as you described it used to work in KDE 3.5.10 (8.04).