This is a tough one because I understand if you got a D600 that has a problem it is very frustrating. But a petition will do nothing, sending in defective D600's will Nikon will respond to its repair facilities reporting large numbers of defective D600's. As far as the cost to ship I understand its a bummer but I bet a large portion of sent in D600's have normal dust should Nikon pay for you to have them clean Your sensor. If you say to me if it requires a major repair Nikon should comp the shipping I would agree with that.
My D600 has only had a few dust bunnies so I Am very happy with my camera. But I do understand those who have defective ones being upset because this camera is so good in many ways you hate one thing making unusable.