TiVo Lovers has an article describing a new feature of the TiVo, Amazon Unbox Video Downloads on Tivo. This service allows you to send Unbox content you get from Amazon (either purchased or as part of a special offer) directly to your TiVo, allowing you to watch it on your regular TV.
Not all content is available to be sent to your TiVo, but that probably will increase as this service takes off. Also, there currently is no way to order videos directly from your TiVo, but with the HME environment TiVo has opened the door to anyone to write an application that can do this.
If you register for the service by April 30th, you get $15 credit for movies and TV shows.