Has anybody bought or rented Movies from the iTunes store? If so, then maybe you'll ask yourself this question:
How long do you think it will take before a "Retina density" movie starts playing after you click the "Rent" button?
Currently, movies on iTunes (720p HD) run between 1.5 and 2 gigabytes for a two hour movie. If you choose to Rent, using AppleTV or iPad, it takes about ten minutes to buffer enough of the movie data before you can start playing it. Not too bad - enough to make some popcorn, go to the bathroom, draw the drapes, and get comfortable.
What happens to the file size if they double the pixel density, from the 1280 x 720 now used in iTunes HD format? There aren't twice as many pixels, there's FOUR TIMES as many. Its possible that compression will bring the file size down some, and the audio portion won't increase. But from a practical standpoint, its going to at least double the file size.
Which means that people renting movies to watch on the iPad 3 are going to have to wait an hour or more before the movie starts playing.
If you made it through that thought exercise, ask yourself this: Do you think NetFlix is going to start offering streaming movies in 2560x1920 format? If they did, how long would THEY take to buffer? What would it do to the Internet usage caps many people now are dealing with?
Ask yourself those questions. And then ask yourself how probable it is that Apple is going to start selling an iPad with 2560x1920 resolution anytime soon.