Integrity issues aside, there is one other consequence of walking away from debt that I haven't seen mentioned.
Not every debt holder is just going to shrug their shoulders and let you off, "reduce your principal", or scar your credit. Many will take you through the proper court channels and get a judgment against you. How would you like explaining to your employer why your wages are being garnished? How would you like it when a LEO shows up at your moms house to server her papers and all the neighbors see?
afchic, according to the Fair Debt Collection Practices Act, if you tell a collector not to call and they continue then you can report them.
http://www.privacyrights.org/fs/fs27-debtcoll.htm