There is a lot of interest about Sentiment Analysis (SA) (also called Opinion Mining) in the press at the moment. Since the last decade or so there has been research in this area because we have made enough advances in natural language processing and AI to allow us to contemplate implementing such a thing in machines. There are a number of commercial systems around and there are also some research ones too. We are seeing that there are limitations and that we do have a long way to go in this area, but we are walking that route and it will be brilliant one day. Right now, as long as you’re not asking for anything tooo complicated, you can use SA as an additional metric in your work. I recommend you follow @communicating because he is the dude as far as SA goes and he will be revealing an interesting little toy later in the year so keep posted.
In the meantime, I’ve written some slides to help you understand how machines deal with sentiment analysis. I’ve kept it maths free (so everything is ultra simplified or massively abstracted) but it should give you a good notion of what is involved so that you have a bit more of a grasp on it. It is an introduction, the topic area is pretty huge and I could write a book on it. I’ve included further resources for you. Enjoy!