Recent article written in The Economist entitled 'Listening to the Internet' provides a comfy introduction to brand reputation tracking online. Here's an excerpt:

"the direct, unfiltered, brutally honest nature of much online discussion is gold dust to big companies that want to spot trends, or find out what customers really think of them. As a result, many firms now monitor online chatter as an adjunct to more traditional forms of market research."

The article cites one weakness of online research in that it excludes non-internet users. Who are these strange halfbreeds? Sorry, couldn't resist.