Warren Buffet's rules of investing: 1) Never lose money. 2) See rule #1. I use municipal bonds to immunize my expenses ( http://en.wikipedia.org/wiki/Immunization_%28finance%29 ). Municipal bonds (affectionately called "munis") are not subject to federal income tax because of a Supreme Court decision in the 1890s. Most states also exempt the interest on their own municipal bonds from their own income tax (of course,…
Risk of default on these bonds has never been higher -- hence the yield. The market has plummeted in the past few months, but don't try to catch a falling knife.
"Default" doesn't always mean that the principal won't get paid back. In almost all cases, it means that timely interest payments won't be made. Even in the most recent bond crisis in Puerto Rico, the "risk of default" was somewhat mitigated by the threat of shutting down the government offices (i.e. put the employees on unpaid furlough) just so the Puerto Rico could make their bond payments.
IMHO, it would be political suicide for a municipality or state to let bonds default. Here's an interesting article on this subject:
http://www.publicbonds.org/public_fin/default.htm
The default in the 1975 New York case was for revenue bonds. I don't know the exact details, but they sounded like LTX bonds.