[currywurst]

The culmination of such an approach is realized in the Beaker Notebook (http://www.beakernotebook.com). You can not only switch between languages, but the environment allows you to share datasets between Groovy, Python, JS, Julia, Ruby etc ..

It's a work in progress, but something to watch out for, imho

[baldfat]

Well this is what iPython is morphing into. iPython is going kernel agnostic and you can actually use dozens of languages already. Juypter is the new name.