Xbox live has an HD rental service that allows you to download HD movies to rent. They are like 6 bucks though.
These movies are around 4-6 gigs. I Think they compress the heck out of the audio and maybe only offer 2 channel sound to compress the movies to 6gigs. Also the movies are all in 720p.
I downloaded a couple of them before I had my axioms so I can not comment on the sound. The video looked great though.
I think that in the meantime a 'water downed' online HD service like xbox live has is a solution for some people; but to allow lossless 5.1 sound plus all of the extra content via download is not practicle due to file size.
What ever HD media wins the 'war' (looks like it won't be HD DVD) will probably be around for at least a little while
