From Roger Lowenstein in the NY Times Magazine: Walk Away From Your Mortgage!. Although Lowenstein doesn't cover new ground, he does provide a nice summary.

No one says defaulting on a contract is pretty or that, in a perfectly functioning society, defaults would be the rule. But to put the onus for restraint on ordinary homeowners seems rather strange. If the Mortgage Bankers Association is against defaults, its members, presumably the experts in such matters, might take better care not to lend people more than their homes are worth.

Strategic defaults (or "ruthless defaults" as they are known in the mortgage industry) are not new. But they used to be pretty rare - and it has always been hard to quantify.