Since the bondholders have voted to reject the emergency manager's plan, which would have paid them pennies on the dollar, the city is now attempting to discharge its short-term and long-term debt. If they get what they want in court, it is likely these bonds will become worthless. Even if they are only able to restructure the debt, its likely that bondholders will need to accept large concessions. However, this may not be immediately reflected in bond prices as it's very possible that the market for these bonds will be very limited in terms of who they could sell them to.
If you were to buy them now , that would be a bet on some outcome other than bankruptcy and the discharge of the city's long-term obligations. President Obama has already stated that he monitoring the situation, and it seems unlikely to me that after all of the support given to the auto industry in the last several years that the federal government will do nothing, if only to avert job losses. However, I think it's likely that state aid will be limited at best, as Michigan's economy has been struggling for a number of years.
There aren't many large precedents to look at for guidance. One of the largest public entities to declare bankruptcy, Orange County, was a very different situation because this was due to malfeasance on the part of its investment manager, whereas Detroit's situation is a much larger structural problem with its declining economy and tax base. I think the key question will be whether the Federal Government will consider a Detroit bankruptcy to be a large enough embarassment/failure to take significant action.