The Zune and Xbox 360 are united by brand now - it would make sense for this to be by content too, purchase a TV show on your Xbox 360 and get the Zune copy or vice-versa. A united marketplace already exists from a server / content delivery point of view so this is not beyond the realms of possibility it is possible technically however it is the TV / Movie studios who decide how their content will be available - for example in the UK I have access to fewer films on the Xbox Video Marketplace (or Zune I guess now?) and no TV shows - all we'll get extra is Sky Player and you can see the content this supplies via
sky.com/skyplayer.