It appears that my decision to send back the D600 was a good idea as shown by this blog post by lensrentals.com: http://www.lensrentals.com/blog/2012/10/d600-sensor-dust-issues After my post about my issues with the D600 I was criticized for returning it and not knowing how or when to clean a sensor. At the same time I emailed all the major camera review sites with my issue asking them to check their test cameras to verify the problem. Of course, I received no responses. I also email Nikon tech support and got the standard how to clean a sensor email back. I replied stating the spots did not go away after multiple cleanings and they suggested I send them the camera to check. Really? I should send it to them and take a chance 2 months later they would say no issues found? My return timeframe would be gone. I don't think so.
So at this point I am waiting for more information or a resolution by Nikon before I make another purchase. Maybe I will look at the Sony or Canon options since I have basically sold all my camera gear and could start fresh. I really did like the look of the images I was getting from the D600 though.