I sent my D600 in to Nikon to have the sensor cleaned. There was an inordinate amount of spots clustered in the upper left corner area of horizontal images. This would correspond to the lower right area of the sensor as you look at it from the front of the camera, since the image is inverted and reversed.
|The red oval shows the most concentrated area of sensor spotting. Click here to download a hi res version of this file.|
|I pumped up the contrast in this image to show the sensor dust more clearly. Click here to download a hi res version of this file.|
Fortunately, the entire process was covered under warranty. Whew!
I hope Nikon is correcting this problem on newer releases of this camera, but I have heard nothing about it. So you might want to check your images for sensor spotting if you have a D600, and have the problem fixed while the camera is still under warranty, particularly if you have an early model, as I did.