We had a half dozen applications stop working today for the same reason. Don't know why, but a quick shift over to json_decode did the trick. Note that pulling the result out of the decoded array changed slightly.
The problem was fixed sometime between 6PM EDT and 9:45PM EDT saving me from revising my code. Thank you Stephen for posting that solution. I wish Yahoo was as helpful as you.
Regarding the change to BOSS and a paid for model starting on Nov 12, 2012 how about Yahoo sending an Email to people using the interface! The conversion from Geocoder to Placefinder took me and many others by surprise, and I'm not in the mood for a repeat performance.
The change was not intentional, and we have reverted it back. The current Placefinder service is shutting down on November 17th but we will continue to support our YQL table for non-commercial applications that are limited to 2000 queries per day per developer. We recommend you transition your code to the YQL table if you are in this category.