Movies Anywhere does this for supported movies (which is not as many movies as I'd like it to be, to be sure). I can buy a movie on Vudu and watch it on Amazon Video or Microsoft's video stores (Windows/Xbox) or I think on iTunes, so long as I link my accounts. It's not quite the vision I have where platforms, not retailers, handle the front end and make it transparent to me what provider is giving me the movie (if I want to watch a movie, I shouldn't have to care if it's on Hulu or Netflix or whatever, I should just be able to pick it and watch it), but it's a step in the right direction.