I'm very optimistic about iPython as the solution for "executable documents", much so over Javascript by itself (despite all of Bret Victor's amazing efforts).
There is a development branch of iPython by Brian Granger that emits JSON when you display a python object in the notebook. You can easily create handlers that display any Python object using javascript. That means D3.js or whatever HTML5/WebGL app you can throw in there becomes the front-end to a very powerful numerical computation back-end. Check out this demonstration I help code, http://www.flickr.com/photos/47156828@N06/8183294725
There is a development branch of iPython by Brian Granger that emits JSON when you display a python object in the notebook. You can easily create handlers that display any Python object using javascript. That means D3.js or whatever HTML5/WebGL app you can throw in there becomes the front-end to a very powerful numerical computation back-end. Check out this demonstration I help code, http://www.flickr.com/photos/47156828@N06/8183294725