I can't find an English link to submit but it's a issue that's been confirmed in France at least http://rezonances.blog.lemonde.fr/2012/09/24/facebook-bug-et-rend-publics-les-messages-prives-de-ses-membres/
Check your wall. For me it was 2009 and earlier.
The worst part is probably the fact that the messages that YOU sent to friends are currently on their wall right now, and there is no easy way that I know of to delete them.
EDIT: quickest way to hide all of your friend's posts on your wall http://cl.ly/image/0s1e093Q0s1m
EDIT: it's now the top story on LeMonde.fr (biggest paper in France) http://lemonde.fr and on TF1 (biggest private TV channel) http://lci.tf1.fr/high-tech/facebook-des-messages-prives-publies-sur-les-timeline-7545127.html It's happening to all of my French friends and a lot of US people too (I've been living in the US for almost 2 years…)