Transparent QWindow with QtQuick 2.0

I tried to create a transparent QQuickView so that i can integrate QML with my own opengl render engine and use qml for user interface part, However i figured out that QT doesn’t suppert transparency for QWindow based classes on Windows 7 for QtQuick 2.0. Is there a good reason why Qt doesn’t support transparency for QWindow on Windows 7? (I didn’t try it but I think it is supported on Linux)