I had the same problem. The only way I found to fix this, was to reimplement the QGraphicsItem::paint() and apply the flags QPainter::Antialiasing | QPainter::SmoothPixmapTransform to the painter.
Like this:
void Rocket::paint(QPainter *painter,
const QStyleOptionGraphicsItem *option,
QWidget *widget)
{
painter->setRenderHints( QPainter::Antialiasing | QPainter::SmoothPixmapTransform );
painter->drawPixmap( boundingRect().topLeft(), myOriginalPixmap );
}