It works on a few Windows 7 Pro machines and does't work on some Windows Embedded Standard 7, here is the reason :
QPrintDialog exec or open function calls PrintDlgEx which require the calling thread to be "Apartment Threaded" - ( CoInitializeEx(NULL,COINIT_APARTMENTTHREADED ) ), if not, the behavior is erratic.

I think it should be explicit in the documentation that the calling thread have to be ApartmentThreaded