QPen default width as been changed following a dev mailing list discussion initiated by Bache-Wiig Jens (http://lists.qt-project.org/pipermail/development/2012-October/007026.html).
I understand the rationale of the change, but i think that the impact on existing code was underestimated, as if no-one was using (implicit) cosmetic pens, or not caring. And NB all graphic items containing pens are affected.
At least, the documentation should be updated, and that change should be part of documented porting issues.