It's a pity that
QApplication::QApplication ( Display * display, Qt::HANDLE visual = 0, Qt::HANDLE colormap = 0 )
is X11 only. I wonder if there is any way on Windows and OS X to create a window which can be drawn to using transparent brush and will in turn be transparent (like with the above constructor for X11 as shown here: svn://labs.trolltech.com/svn/graphics/dojo/argb).