I know this thread is a little stale now, but in case anyone ever comes along and wonders the same thing I managed to achieve what I wanted by creating a script that runs when kde 4 starts with the following in it:
Code:
qdbus org.kde.kontact /kontact/MainWindow_1 com.trolltech.Qt.QWidget.close
Kontact now pops up briefly when I log in, this script kicks in and kontact is then automatically minimised to the taskbar.