The folks at LensRentals have discovered something about the Nikon D600 — it seems particularly prone to dust spots. With most of the cameras they rent out, about one in four require a sensor cleaning, but all 20 of the Nikon D600 needed to be scrubbed. The sheer number of cameras they interact with is what makes their observations so interesting. You can see in the image below what happened when dust images were taken from each of the cameras, and then combined in Photoshop to show the dust locations. The left corners and top were all consistently getting grimy, even scaled down to this scale.
They theorize that this problem may be related to the shutter curtain in the D600:
We aren’t absolutely certain about the cause, but when we have to look at the sensors for all this cleaning one thing is quite apparent. The D600′s shutter curtain opening seems a bit larger than the other Nikon cameras with a bit of a gap around the shutter curtain. It may well be the shutter movement is pulling dust onto the sensor.
If you have a D600, keep an eye out for dust on the sensor, it sounds like this might be a fairly widespread problem. And if you want to try and do something about it, here's how LensRentals recommends cleaning a sensor. They think the dust will stop accumulating, but only time will tell.