In the most recent episode of NPR's Planet Money Podcast, they interviewed a real estate lawyer in Arizona who talked about the large numbers of her clients who have been asking for advice about doing a "strategic default" from their mortgages. (Like most of Planet Money's episodes, it's very interesting and well worth listening to the whole thing.)
For example, suppose you borrowed $500K to buy a house during the bubble, and now, a year later, identical houses in the neighborhood are selling for $300K. You don't want to keep paying the $500K mortgage for a house that's really only worth $300K now. So, you go buy another house in the neighborhood, taking out a mortgage with your good credit. Then, you move into the new house and simply stop paying the mortgage for the first house. Eventually, the bank forecloses on the first house and takes it away, and your credit rating takes a substantial hit, but you save a ton of money.
Refusing to pay off your mortgage is a breech of contract, which is not illegal, according to this lawyer, and at least in Arizona, the bank can't even sue you to recover what they lent you other than by taking the house. (They can't try to garnish your wages, take your other stuff, etc.) So, for many people, this is a sound business decision.
They discussed the ethics of this on the podcast a little but of course didn't come to any definitive conclusions.
We could all take turns spouting about our ethical intuition about this issue, but I'm really only interested in hearing about relevant sources in the literature. And of course, if this issue affects you practically (which it doesn't me, thank God), make sure to consult your own Rabbi and lawyer before acting on any information presented here.