This is exactly what's going on. They're making the bet that they'll be in demand later and able to pay off their loans. Just like buying a house and being able to make money from it later. After all, everyone had a rationale as to why the price of their house would increase just like they do about the job market turning around for people with a degree/diploma in X.
Part of the collateral for student loans is, technically, your economic freedom. It's easier for banks to assume you will pay the principal of your debts, when government is the acting collection agency. It's very difficult to escape garnished wages.
However, if something changes and people have a way to shed their student loan debts, I think the bubble will pop very quickly.
Is this a reasonable bet to make? What is the earning potential of someone with a college degree? Isn't it something like a $1M+ increase over one's lifetime?
WSJ had article on student debt recently, with lots of data. Median debt load was 18-19k USD [1]
If it were all via Federal Stafford loan, the interest rate would be 3.9%. If the payback period was 10 years, the monthly payment would be a whopping $191 per month.
[1] http://online.wsj.com/article/SB1000142412788732443200457830...