Measuring Twitter impact

There's an interesting debate on measuring the impact of Twitter.

Our experience is that not many companies track their products on Twitter. There isn't currently the volume to justify it. However, there's a growing case for doing so because:
a) Twitter is an incredibly useful tool for SEO (see Newscounter's Twitter page.)
b) Opinion formers use Twitter
c) An increasing number of journalists use Twitter personally and for new content

See some of the discussion here.

Do you know anyone who uses Twitter to monitor their brand reputation?