I have had 2 d600's both went back to the vendor they agree with me I paid for a new camera that works, not one with a problem. Right now I am waiting for a new shipmet and they will be sending me my third one. I posted a picture ( over cooked a bit) its in my gallery both of the cameras looked like this after going out and shooting 800 or so photos, with out changing a lens. I use the 24 to 70 on my D 7000 with no problems.
My vendor said more then 1/2 of thier first shipment came back with dust problems so I think it's more then just a few bodies. I also think it has to be coming from the shutter it's self. And my guess is in a year or so there will be reports of pre mature shutter Valkyries in the D 600. Just myn2 cents.
Besides he spots I loved the camera for the short time I have had to shot with it. It takes photos with great detail with good dynamic range allowing for some nice post processing.