I just recognized that QSplashScreen doesn't work correctly with setting setAttribute(Qt::WA_TranslucentBackground)
... I tried to show a splashscreen with a *.png (with transparent corners)
... when I set TranslucentBackground it is just a black rect

QPixmap pm(":/resources/images/splash.png");
QSplashScreen* splash = new QSplashScreen(pm,Qt::WindowStaysOnTopHint);
splash->setAttribute(Qt::WA_TranslucentBackground);
splash->show();

Atm I can only test it on my WinXP, so I don't know if it is something OS depending or general ...

If I do something similar with the same *.png and QLabel, it works as expected:
QLabel* testLabel = new QLabel(f,Qt::FramelessWindowHint|Qt::WindowStaysOnTopHint);
testLabel->setAttribute(Qt::WA_TranslucentBackground);
testLabel->setPixmap(pm);
testLabel->show();