Here is a video I found from Stephen Silver, a London born cartoonist and character designer, about doing work for free. He speaks about the situation of big companies hitting up starting up artists and students to do work for them for free. Take a look at the video to hear his opinion on the subject.
He makes a good point to side the profession of artists with more classic examples of a profession that provides a service and relate asking an artists for free work to trying to get your pipes fixed for free. The thing I take away from this video is that every opportunity needs to be approached from a very analytical view.