On 7th of Nov. I did as post which related to HD downloads onto Xbox 360 via XBL service that was set for launch on 22nd of Nov. and I would like to ring a bell that today is 22nd of Nov. and today onwards you will be able to access to the downloadable services.

As informed earlier, you would be able to download HD loveliness of about 1000 hours will be available for downloads on 20GB HD drive. The announcement also states that the HD movies will be downloadable on rental fee while shows will be available for sale.
Xbox 360 holders will be able to go onto the XBL service and buy both movies and TV shows and enjoy them on their Xbox 360s. With HD content, SD content will be downloadable as well. However Microsoft didn't mention the exact price format for the service downloads, anyway your curiosity ends today as the service is inviting you to access.
Via: kotaku