Just putting in my two cents as a recent intern/apprentice. I had the opportunity to intern under two different entities: the first a music producer/live sound engineer with whom I became close friends and that, as I began to learn, made a point of paying me on the rare occasions that my presence involved more actual work than learning. It was my first time ever in a sound related enviroment, a great opportunity, I made friends and business contacts and most of all I learned a lot.
My second internship was for a small TV production company, and it started out fine: I was prepping stuff, having access to their one-man shoots and getting to learn stuff from their regular sound guy, I was able to pop by their post place and just chat or actually get my hands on stuff, etc. Soon, however, I was invited to record for them, with their equipment, and very quickly I was recording/doing post for an entire big-brand sponsored Internet show - all of this for an insignificant fee even for my country standarts. I talked to them and told them: either we go back to me actually learning stuff or you pay me properly (which, honestly, I wouldn't mind, but made no sense: I was at a stage where I should be learning, not mixing on my own!). They, of course, "fired" me, and all was explained later: they low-ball their prices, and rely on interns to get the job done.
I still have a friend there, who just edited a feature documentary for them for a monthly fee smaller than that of a regular editor's weekly fee on my country. The reason? He's learning and getting to make great contacts. ---
In short: being an intern/apprentice can be an amazing thing. But it has to, as soon as possible, be subjected to some sort of regulation. Companies all over Europe are abusing it, and it seems like film productions companies in the States are as well.
edit: sorry for the long rant!