I really like the ideas expressed by Tobias Mayer in his post “How to Hire an Agile Consultant.” I recommend you go read his post.
I found Tobias via the comments on this anonymous “editorial”. That happens to be a post with some good ideas and concerns as well, but I have to fall in with the commenters who take it to task for its anonymity, in spite of using the personal pronouns “I” and “me” consistently throughout. Good ideas, but no-one to whom one can give credit.
I haven’t yet directly experienced the coach = scrummaster situation yet, but I can easily see how it will happen as more and more companies look for help in “doing agile.” And more and more companies spring up to offer these services.