I had a discussion with Nikon repair facility in Canada about this recently. They told me Nikon has realized that the D600 seems to attract more than the usual amount of particles on the sensor filter, and for this reason Nikon has decided to include free sensor cleanings during the camera's warranty period.
Not everyone lives close to an official Nikon repair facility, so if you don't want to ship your camera off and wait for the repair, you either find a shop close by that you can pay to do it, or clean it yourself.
I think that's the best offer we're likely going to see from Nikon.
Bought a refurbished D600 from Cameta (SN: 3008xxx). I was speaking with their salesperson so I can't verify its validity but the guy was saying that Nikon was working in their "fix" but couldn't verify if the batch they recently got had that fix. The camera started with ~380 actuations and I'm about ~450+ now with no dusts so far. Hoping the trend continuous...
-=edit=- Well...I spoke too soon. I went ahead and did a dust hunt and I found some. Also concentrated on top left corner + on mirrobox D:
Anyone know the lead time for getting sensor cleaned at Nikon Service?