Most HD movies (just the movie) are running closer to the 20GB side of that spectrum than the 50GB side. That is why most Blu-Ray movies are only single layer right now.
Downloading HD movies would be painful for a while. I guess that you could always set up a queue system (like NetFlix uses for actual discs) and then with a set-top unit with a hard drive have it download content to the box for the currently available movies from your queue, plus the next 1 or 2 movies but have them "locked" until the first movies in the queue are deleted.
Oh wait, that IS what NetFlix is planning for later this year with LG. Of course, that was when LG announced that they would would add this feature to their next Super-Blue player so it would do Blu-Ray, HD-DVD AND NetFlix downloads. NetFlix said that they would increase the number of movies available for download and work "towards" HD movie downloads.
