Here's the work I've done on sentiment analysis in R.
The code is, by no means, polished or well-packaged, but I posted it on Github with basic documentation. I used the ViralHeat sentiment API, which just returns JSON, so the actual function to do the sentiment analysis is pretty trivial (see code here).
Feel free to contact me if you're having trouble using it. And note that you'll need to register for an API key with ViralHeat before you'll be able to use it. If you're finding the quotas too restrictive, I had contacted them and they were happy to give me a ton more queries for a few months while I played around with the API.
