Following up on Anil's post, Life on the List comes HARD DATA showing that having a lot of followers doesn't tell us much about influence. From the abstract:
We make several interesting observations. First, popular users who have high indegree are not necessarily influential in terms of spawning retweets or mentions. Second, most influential users can hold significant influence over a variety of topics. Third, influence is not gained spontaneously or accidentally, but through concerted effort such as limiting tweets to a single topic.