Since the Nikon D610 was announced as the solution for the oil/dust issue present on many D600 cameras, I have received several reports from readers (mainly from Europe) who were able to get a new D610 camera from Nikon as a replacement for their "dirty" D600. In some cases (I believe it was France), a small fee was charged for getting a brand new D610 camera.
I recently received a similar report from the US - see the above screenshot. I have no idea how Nikon determines who can get their D600 replaced with new D610 camera. Maybe if you use it professionally or if you have sent it already too many times for sensor cleaning - I really have no idea. The standard procedure for the past few months has been for Nikon to replace the shutter mechanism in every D600 they receive.