For quite some time there has been plenty of uncertainty in the muni bond market regarding the financial health of muni bond insurers. This has led muni bond yields to increase (to compensate for the additional risk) and prices to fall. This in turn has led some contrarian investors to snap them up as a 'good deal'.
Traditionally, munis have been considered relatively safe because most of them are backed by the taxing authority of municipalities. Right now, municipalities are cutting staff and expenses as tax revenues fall, but I don't see much risk that they will begin defaulting on their financial obligations. In an economic climate like we have today, questioning all assumptions may be a good way to stay alive, so I'm keeping a close eye on the muni market.