What I have noticed is that the highest rate of return goes to those who borrow the most (top-tier business schools, med school, etc.) and those who borrow the least (trade schools).
Those stuck in the middle are the ones who still borrow a lot of money and go to a fairly expensive school, but end up with a degree that has a salary ceiling of $50K/year.
Anyway, it seems to me that eventually the "middle" group will realize that the "college experience" isn't worth the price (relative to the potential salary), so you'll see more and more kids going into trade schools or community colleges.