Motivated by the excellent iPython notebook environment, I wanted to create a similar experience for R users. Yihui had already designed a Shiny app that let users knit an Rmd file in the cloud. However, it had to be taken down due to security concerns, as a user could potentially run any malicious code in the Rmd file.
I took Yihui's code and split it into two parts. The first part is an Rmd viewer that allows a user to view an Rmd file and the rendered HTML using a gist as source. Unlike iPython notebooks which only require an .ipynb file, the Rmd viewer requires both the source Rmd file and the knit html. This is not a deal breaker in reality, since it is possible to write wrapper functions that automatically upload the Rmd file and the rendered html as a gist (or as a github repo). Another approach to achieve a single source file would be to hide the Rmd file inside the rendered HTML using a script tag with text as MIMEtype, which can then be read by the viewer application.
The Rmd viewer allows a user to view the Rmd source and the rendered HTML side by side. The download button in the navbar allows a user to download the Rmd file to the desktop. Having downloaded it to the desktop, this Rmd file can be opened as a Shiny app, which constitutes the second part. The Shiny app has a UI very similar to the Rmd viewer, with the difference that a user can edit the Rmd file and render it using the Knit button on top.
This is still proof-of-concept and I will be putting it up as an R package on github shortly.