I will 100% agree that Vudu is the 'killer movie app' that is currently missing, with Amazon being #2. However, I would actually prefer an alternate approach. When we purchase movies from Apple they get a cut of the profit then pass along everything else to the studios. IF Apple was to join the Ultraviolet consortium (yeah... I know... when hell freezes over) then it simply wouldn't matter. VUDU wouldn't matter because Apple would be the single point of front-end access for all my movies. On other devices Vudu would work just as well. And either way I would have access to a full library.
1. Apple would still get a cut of the sales
2. Apple would still own the user experience on Apple devices
3. Users/Customers would have an even bigger reason to purchase through Apple
Everyone wins!