In order to show / hide the sub window containing a custom widget, we have to subclass the slots show() / hide() in our widget and explicitly call the sub window slot.
In Qt3, this was not neccesary. I think this is a bug in Qt4. When you pass the window flag Qt::SubWindow in QMdiArea::addSubWindow(), it should be implicit that show / hide slots of the widget connect to the sub window show / hide slots.

Example in my widget::hide()
QMdiSubWindow subWin = dynamic_cast<QMdiSubWindow>(this->parentWidget());
if ( subWin )
subWin->hide();
else
hide();