Here is http://websvn.kde.org/?view=revision&revision=406411 a commit by Aaron 
Seigo after which the buttons on the Kicker's taskbar ceased being visually 
pushed (in classical mode). 

Here is the key string in kicker/taskbar/taskcontainer.cpp:

before:

style().drawPrimitive(QStyle::PE_HeaderSection, p, QRect(0, 0, width(), 
height()), colors, sunken ? QStyle::Style_Down : QStyle::Style_Raised);

after:

style().drawPrimitive(QStyle::PE_HeaderSection, p, QRect(0, 0, width(), 
height()), colors)

I restored the missing parameter and now it works well.