the R community has recently & rapidly been building a notebook environment similar to that of the iPython notebook.
I've had several conversations with R folks who insist that RStudio + Knitr does everything the IPython notebook does, and I feel like I've had a hard time conveying why I like the notebook for data exploration, prototyping, and demoing.
E.g., the R notebook linked to below looks neat, but it's still just markdown compiled to a static document. That's a different workflow--not a better or worse one; but certainly different.
Personally, I find it more interesting to mix languages in a single notebook - we have a %%R cell magic which allows you to have R cells in amongst your Python code, passing data between the two. An R kernel would only run R code.