The announcement of Nielsen Buzzmetrics has attracted attention to the applications of Opinion Mining (and/or Sentiment Analysis), a relatively young field with huge potential applications to Marketing Research. Opinion Mining is about automatically getting to know what people think on an item (brand, product, anything) from their explicit opinions in blogs, comments to blogs posts and news stories, Social Networks, elsewhere in the Web. It involves Natural Language Processing of texts in order to identify the polarity of opinions (like, dislike, neutral), and many more.
A number of corporations have been offering services/consulting, and software (possibly as a part of a complete data/text mining solution). Some of them are mentions in this post: "Sentiment analysis has more legs than we'd bargained for." I know about at least two Yahoo! Groups in which there relevant discussions: SentimentAI and TextAnalytics.
From a research perspective, the survey by Bo Pang and Lillian Lee for Foundations and Trends in Information Retrieval is more than a must. Just check it out for tools and datasets, if you want to get into this challenging field:
Bo Pang and
Lillian Lee(2008) "Opinion Mining and Sentiment Analysis", Foundations and Trends in Information Retrieval: Vol. 2: No 1-2, pp 1-135.
A note on the link: it works sometimes (at least for today...).