Take the above Flipable element. It has on both of its sides a Rectangle with 'clip: true' set and inside that a visible element which doesn't exceed its parents boundaries (in other words, there is no clipping required). If you rotate the above flipable beyond 45 or -45 degrees artifacts will begin to appear at the same size and shape of the clipping rectangle at the particular angle. This shape will then remain on top of everything else, leaving a small hole in the middle where the rendering shows through. You will notice the same behaviour at around 135 and 225 degrees (45 and -45 degress for the other side).
If you increase the size of the content so that the clipping rectangle has something to clip then the artifacts go away.
You can remove the outer rectangles from from each side and you will still get the same issue – they are just there to provide some colour so you can see the artifacts more clearly. You can remove the behaviour on rotation and the issue still shows, and you can also remove one of the sides and the artifacts will still appear (though of course will only be triggered by the side with content).
Jari Airaksinen's video shows the situation pretty well too – this is the exact issue I'm having.
This was working fine in the Qt SDK 1.1 Tech Preview (Qt 4.7.0?) but has shown up in the Beta release (Qt 4.7.2 final). I haven't had a chance to test it on Symbian^1.