Back in March Nikon officially promised to replace defective D600 cameras if the spot issue is not fixed even after the warranty expires (the company allocated $17.7 million for D600 related issues). Here is an email I received from a reader that describes the process - note that the D600 can be serviced in any country, regardless where it was purchased (new D600 cameras are still available on eBay):
Ok here is how it works: If you have a Nikon D600 and you want to get a new D610 you need to go to Nikon service centre or simply ship it, they will change the shutter and clean the sensor for free. Now here is the interesting part, after the third time when you return the camera(D600) for sensor cleaning you have an option to request an exchange for the brand new Nikon D610, again they will not offer it to you but you can ask them after the third time and they have to change the camera for you, obviously only if the problem of the sensor dust occurs in all 3 times. However the Nikon D600 has unlimited free sensor cleaning forever in any country around the world. Also to change the D600 for the D610 or simply only to change the shutter mechanism it is free in all the countries regardless where you both your camera. An example: I bought my camera Nikon D600 in Australia and I had 3 times sensor cleaning in USA after that again in the USA they exchange my D600 for the new D610.
Hope this helps people who were having many issues with the sensor dust and still have it after many cleaning.
The bottom line, many people refused to exchange their D600 for the new D610 because there is not too much of the difference as with the D600 they get free sensor cleaning for life, in other words you save at least $100 -$150 a year for the sensor cleaning.