We tend not to get too excited about sensor dust problems here; we clean sensors on every camera after every rental, so it's just routine. When we started carrying the Nikon D600 they all arrived with a fair amount of dust, but that's pretty routine, too. Manufacturing and shipping can be a dusty experience.
When our techs started complaining that D600s were all coming back from their first rental with a lot more dust (despite being freshly cleaned before leaving) we didn't pay much attention to that either. We all remember the oil / dust issues the D3x and D3s had. Those mostly cleared up after a few cleanings.
The dust kept reappearing with every rental, and more impressively - it was generally in the same location (upper left 1/3 of the image). That did get our attention, so we started looking into the matter a bit. We kept dust pictures for 20 consecutive D600s returning from rental and saw the problem was very real.
In general, about 1 out of 4 cameras requires sensor cleaning after a rental. All 20 of the D600s did.
Here are a couple of typical examples (f/16 blank wall photos, contrast and exposure increased). I would point out that these are downsized to 800-pixel wide images. What is barely visible dust on this is quite obvious on a full-size image. The large specs on this are quite huge at full size.
It's probably pretty apparent that the dust is mostly on the left side of the image. I took all 20 images and layered them onto one in Photoshop, using 'darken if' to show the pattern of dust from 20 cameras.
Again, with these downsized images, only the really large dust specs are showing up, but then, those are the ones most likely to show up in a photo. I would also add these almost all seem to be dust specs, not oil, since most of them can be blown off or stamped off using a Dust-Aid. They don't require wet cleaning to remove as oil spots do.
There are, however, a number of comments from experienced photographers that are having oil spots on D600s. I'm not certain if they're seeing the same, or a different phenomenon.
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.
The real question is: Will the dust eventually stop accumulating on the sensor? I think probably so. There's some dust inside the camera that is getting blown out during early use through the shutter opening. But that's just an educated guess; only time will tell.
For now, though, if you rent a D600 we suggest looking fairly frequently for dust accumulation. We clean each one before it leaves, but given the amount of dust they return with it's fairly obvious there could be dust dots on some of your shots if you stop down to f/8 or more.
NOTE: I know someone will want to know. All of these cameras were from SN 300xxxx or 301xxxx. We have another 40 D600s but since they, too, are from these SN runs I don't plan any further comparisons. We'll check again when we get higher SNs.
Addendum July, 2013: I tire of being misquoted by fanboys on various forums, so please, if you want to quote me, copy paste instead of putting words in my mouth.
1) We found, over time, the dust issues decreased. Somewhere around 5,000 to 10,000 shots the D600s stopped having megadust.
2) When D600s do have dusty sensors, as all cameras do, it still seems to congregate in the upper left side of the image, but it's more normal dust and fewer huge chunks.
3) We have NOT. I repeat, we have NOT ever said newer bodies don't have the problem. We have no idea. Nikon demand is down hugely this year and we simply haven't had to buy any more D600s or D800s (or anything else Nikon, really). I would assume Nikon has figured out a fix by now, but that's just an assumption with no data to back it up.
Roger Cicala, sensor images courtesy Adam Remsen and Scott Rambin