Dave, Maybe not the mortgages, but the houses they purchase, could be made out of corn. A colleague e-mails:
Some years ago an entrepreneur developed a process to fashion modular walls for homes out of corn stocks and other cellulosic waste materials from agriculture. (Karl Hess even lived in one for awhile and advertised it.) Of course building permits blocked inexpensive straw houses (which were safe and very cheap).
Meanwhile, I have a piece about ethanol and other Really Inconvenient Truths (gee, sounds like a great title for a book!) over at Human Events.