Is it immoral to walk away from an upside down mortgage?

Let's say that you owe $500,000 on the mortgage for a house that, because of the crumbling real estate market, is now worth $200,000.

You can afford to make the payments -- but it feels stupid to write a check every month to make interest payments on a loan for a property worth so much less than you owe on it.

What should you do? In a controversial paper, University of Arizona Law School professor Brent T. White argues that you should stop paying the mortgage, hand the bank the keys to the house, and move on with your life.