I stopped taking orders from them in mid-October because of payment issues and I know of at least one other abstractor who had the same problem.  They WERE a very good client until approximately mid-2009, when their former clock-work payment started slowing just as you described.  Everyone that has had a problem with them has gotten the same response, i.e., shock that you have not been paid, this is the first time we have heard anything about it, etc.  After days of dodging my calls and not returning voice mails, I then got the excuse that checks had already been sent, etc.  Interestingly, while one person was telling me this on the phone, someone else called and left a voice mail that there was an error printing the checks and all checks had to be reprinted.  When I did get the check (several days later), it was for only a portion of the amount due me.  I advised them they were going to have to correct their delinquency and warned that if they did not, I would be forced to start rejecting their orders.  When they did nothing whatsoever to remedy the problem, I started rejecting all orders from them.  Approximately a month later, I received a very short letter from their vendor manager under the letterhead of another company, stating that they had removed me from their database at my request and appreciated the work I had done in the past.  Though I hated to lose their work, I have gained some new clients and remain busy and, as you say, have not lost anything by shutting them off since they were not paying anyway.  I would rather spend my time working on making money with good clients than spending additional unpaid hours attempting to collect money that I have already earned and cannot collect.