I'm on my third D600 now, the last one did not come from Costco, I'm about 400 shots in and so far there is no dust at all.
I am glad to hear yours is good so far. I'm on my 2nd D600, and with 507 shutter actuations, there are spots splattered on the left side and the top area of the sensor.
Attached is an unedited macro shot that when enlarged, the spots show up. I took this from this particular vantage point because it offered a light-colored background, and I wanted to see if anything showed up on the sensor at f/29 (and I used the camera's sensor cleaning option right before taking the flower photos). The spots are more numerous than the last time although smaller in size.