QWidget::showFullScreen does nothing on Qt5.0.x under Mac X 10.6.
Example code:

int main(int argc, char *argv[])
{
  QApplication a(argc, argv);
  QWidget p;
  p.showFullScreen();
  return a.exec();
}

This code works OK:

• With Qt5.0.0, 5.0.1 under Mac OS X 10.7 and 10.8
• With Qt 4.8.4 under Mac OS X 10.6.8

This code does not work as expected on Mac OS X 10.6: the window remains non-fullscreen.