Movies are going to be like music. At first not many people downloaded music as broadband became more common, downloading music became popular. Most of the people I know have not bought a CD in years.
For right now xbox live is the only place that lets you download HD content (as far as I know)
The XBOX360 is on the right path... xbox360+hdtv+vista media center = the way things should be.
I use vista media center to record OTA HD content. Play it through my xbox360 on my HDTV. Also use it for playing a massive mp3 collection. This is seamless and vista media center looks very sexy on a big screen. There are no subscription fees, no content to buy.
Xbox live offers HD content for download. I tried out one HD movie (right now movies are only available for rent, tv shows you download you can actually keep) it is 720p and the file was 5gb. Took a few hours to download (was not timing it, but felt like 3-5 hours) PQ was very good... movie sucked (The Wicker Man) You can actually can start watching movies before they finish downloading.
As soon as we get to the point where we can start watching movies as soon as they start to download, downloaded HD programming will gain much more acceptance. Once we can download a HD movie in less than 2 hours, we are set :-)