Freelance editor Katharine O'Moore-Klopf pointed me in the direction of this really depressing article from the New York Times. To sum it up, Inkwell Publishing Solutions did a lot of editorial work for Harcourt's textbooks. Now that Harcourt is in trouble, it's not paying its vendors. So Inkwell has gone belly-up, leaving approximately 50 freelancers scrambling for their very survival.
Something similar happened here in Indy about six years ago. A packager declared bankruptcy and many freelancers got just a fraction of what they were owed. (Somehow I got lucky and got my money just in time.) The lesson we all learned is to never let a client owe you too much. If they owe you money and it's more than a month overdue, you might have to refuse future assignments until you are paid. You have to go with your gut, of course. You don't want to risk being seen as a troublemaker. But you also have to be able to minimize your losses in case the worst happens, as it did for the Inkwell people.