I'm doing a site for a client now that wants to sell films in form of DVD and streaming video. I'm building an interface with a list of videos the customer has paid for (using the order database with order_status system to see which online films the customer has in it's collection).

So there will be a page with list of films, with a link to another custom page that implements a FLV player with secure streaming URL (done with a clever timestamp system, so that you can't download the film directly).

This will not use the zencarts way of handeling downloadable files etc, but a totally custom system.