Whoever moved from Qt 4.0 to 4.1 typically realized that for some widgets, the background is not what it is supposed to be. In the latest Qt Quarterly article Transparent Backgrounds in Qt 4.1, Andreas Hanssen explained this feature in details. He also elaborated a bit about backing store, also a new thing in Qt 4.1 .
Still related to backing store, Trenton Schulz showed how to create Fading Effects with Qt 4.1.