So I decided to have a look too. I've searched for the code on the web and found it here:
http://www.koders.com/cpp/fid93B09A1...+mdef%3Ainsert

(To be honest I can't find it in Linux)

I think I see what you mean, but there still there is something that I don't understand.

I was calling QGraphicsItem::mousePressEvent() in my reimplementation of the mousePress and that event did work, however according to the code here if my item is neither moveable or selectable it should also be ignored, what am I missing?