Tax frees that are backed by dedicated income streams, e.g., real estate taxes (schools) or water and sewer revenues, should be o.k. Those that are general obligation bonds of cities and other municipalities are higher risk.
Fortunately, for those holding bonds before the crisis have seen early pay-offs because the issuers have been able to refinance at lower rates.
Take the city of Chicago. They had ~80K foreclosure filings and ~80K notice of defaults for 2010. Real Estate values are off ~30%. That is trouble for the Windy City.