http://www.eff.org/deeplinks/2009/03/apple-adds-still-more-drm-ipod-shuffle
EDIT 2009-03-18:
It would seem that Apple has not restricted so much as made the documentation available only to those who wish to properly license the “Made for iPod” certification mark on their products. Assuming nobody C&Ds independent manufacturers of headphones who reverse engineer but don’t infringe on the certification mark, I’m not against this. Sorry for the alarm.