The systray intergration for KDE 3.x don't follow the standard for for KDE apps, as aMule uses double click on the systray icon to minimize/maximize while KDE apps uses single click. I can't find any setting to change this either. It's quite cumbersome to have one app in the systray with different behavior from the others.
(Mandrake 10, with 2.0.0rc5 RPMs from amule.org)