At the Elm Workshop 2013 ( https://www.youtube.com/channel/UCzbnVYNyCwES9u3dqYZ-0WQ/vid... ) several interesting features like this one were demonstrated. Many of which were brought to proof-of-concept level in 1-3 man-weeks each.
I'm hoping more people in the browser world come around to the idea that concurrency and parallelism aren't so tricky if you start with language design. Passing immutable objects cheaply between web workers would be a huge deal for projects like Elm!