So if you check out Mike Chon's nice article about Six Attributes of the Good ScrumMaster and to me those attributes look like pretty much same attributes what a good leader should have. As a Scrum Master you lack old school management power, but good leader does not need it. Leadership comes from somewhere else, than artificial power.
Role of the Scrum Master (at least in our company) has been really vague. There are people from old Command & Control to people who are doing bare minimum which is basically calling up mandatory meetings in Scrum. What I think and how I teach is that Scrum Master is a leader and Scrum Masters must practice and learn leadership and they must be also supported by management to do that.