Here's my setup. I purchased a HD movie (my first ever!) from the iTunes Store onto my Mac mini. iTunes on this Mac mini is configured for Home Sharing. Now, when I launch the Videos app on the new iPad, it lists my Shared Library correctly together with the movie that I just downloaded. However, when I tap the Play button, the screen goes blank for a moment before displaying a "Cannot Open" error message.
I am bedazzled by the fact that I can't play this very same movie on my HDTV which is connected to the Mac mini via Mini DisplayPort to HDMI cable (HDCP error, but that's another story for another question), I'm now totally flabbergasted that I can't even play this movie via a channel that Apple has been shouting about.
I tried playing the movie on my MacBook Pro and it works correctly, therefore the file is not corrupted. I am also sure I can sync the entire movie (all 7GB of it) into my new iPad, and watch it from there, but why?