Nikon has a widespread issue of dust/oil on the sensors in about 90% or more of the new nikon D600 models. the actuation of the shutter is flinging oil onto the sensor and you end up with dark spots only visible when shooting a white wall or "THE SKY" i.e. ME . I am not a pixel peeper and I miss film. Here are the facts:
1.I have a new D600 (one month) and I have this issue.
2. the spots show up at f/8,f/11,f/16 (smaller apertures) on the left side of the sensor.
3. some spots stay in the same place and others appear even after a good blowing (with the rocket blower)
4. can be retouched in post processing (about 10 -15 spots)
If anybody's in the market for a D600 i would advise to hold off a bit and see where this goes. other than this issue I love the camera. you can see samples of this problem on the internet.