Joshua Sinel of Kazaa has built a business around this idea. He gave a talk at one of our roundtable meetings a while ago. It was very early in the cycle of using unstructured content from communities to gauge sentiment, but he showed how they could do it. Not sure if they are still in business, but there should be content out on the web to help you learn more. There big client was Martha Stuart's Omnicom where they were constantly reviewing content for brand awareness and loyalty. A second client was one of the AOL channels. So, they have some experience with doing this for large communities with a lot of data.