It might be interesting to analyze how it is you serendipitously find new films and whether that could be applied to a site like Netflix. Is it a cover that catches your eye? A recommendation from a passerby? A random pick off the shelf? Etc. Any of these approaches could be replicated in a web interface.

I think Stumbleupon was quite clever at what it did and I'm disappointed more services haven't picked up on the "Give us a broad category and pull the lever to make the slot machine spit out something you've never seen before" approach to content. But it does give me hope that algorithms can provide the experience of serendipity if the people designing the application make that choice.