David Brooks makes several good points while asking the basic question of whether the cost of college is worth it. Money quote:
This is an unstable situation. At some point, parents are going to decide that $160,000 is too high a price if all you get is an empty credential and a fancy car-window sticker.
Of course, if you are pricing Duke University for little Johnny, you recognize $160,000 as a bargain (the cost of attendance for Duke is $59,343 for 2012-13; our expansive financial aid policies are contained in the link).
I think the practical definition of a bubble is people are rushing to desperately spend their money on something up until the moment where almost no one is willing (or able) to do so. You don’t really know it was a bubble until it pops, but you can likely get some hints. This chart from mymoneyblog may be a hint of sorts.