I've just read an interesting article in the New York Times about unpaid internships. I absolutely agree with all the criticism toward for-profit employers who are gaining benefits from interns' work and do not pay them. The article draw a distinction between educational and non-educational internships, the latter often include menial clerical work that teaches interns nothing and does not help to develop new skills. Another issue, raised in the article is based on the fact that intern are often not considered "real" employees, so many internal rules and policies do not apply to them. The author cites a case where a sexual harassment accusation by an intern was dismissed because she was not an employee. Furthermore, only more affluent students can afford to work for free for several months, especially in places like NYC or San Francisco.
http://www.nytimes.com/2010/04/03/business/03intern.html?bl ETA (2006 op-ed on the same topic):
http://www.nytimes.com/2006/05/30/opinion/30kamenetz.html ETA2 (a story about people paying to get an internship):
http://www.nytimes.com/2009/08/09/business/09intern.html