There's an interesting piece in the current edition of Marketing about responding to blog postings by Robert Gray.
Tetra Pack responds to inaccurate postings on blogs. It seems a good idea. Too many companies are nervous about replying to blog posts in my experience. They worry that they will come across as too corporate and patronising or that they will provoke a storm of protests. But organisations have a fair right to reply and providing the replies respect the online environment and you are open about who you are, it's a good policy. Besides people tend to behave better when they know they are being watched.
Obviously it is useful to know which blogs are the most influential in your sector. Which carries the most weight. As the replies to those will go further.