My D600 came back today. It was gone for 11 days, including 2 weekends.
The repair invoice says: RPL SHUTTER MECHANISM CLN LOW PASS FILTER GENERAL CHECK & CLEAN
UPS delivered the camera to Nikon on a Friday afternoon, Nikon handed it back to UPS the following Friday. They had it in their process for 5 working days.
In case anyone cares, my D600 is a refurbished unit. I sent it in 2 weeks before the end of the 90 day Nikon warranty.
Funny story: I grabbed the nearest lens to take a test pic. It happened to be my Tamron Adaptall 2 SP 90mm f/2.5 Macro with the 2X teleconverter attached. I took a shot of the sky at f/64, and there was a HUGE blurry blob near the middle of the frame! Of course I panicked, but decided to try a more reasonable test with the 50mm at f/22. Nothing visible. Whew! Further inspection revealed a fairly large speck of something on the front element of the TC which the blower bulb easily removed.