Nikon cleaned the sensor as part of the servicing, and the artifact only appears at fast shutter speeds, so it unlikely to be a physical problem with the sensor itself. My uneducated guess is that it is a problem with the shutter - in fact, after much searching (this appears not to be a common problem) I was able to google a similar issue someone had with a Canon EOS 30D - a semi circular over exposed crescent in all shots at fast shutter speeds. In that case, it was suggested that this was the result of a problem with the synchronous operation of the shutter blades (http://photo.net/canon-eos-digital-camera-forum/00LS88).
Upon calling Nikon after I received my camera and found it still had the same problem, they requested I send some sample NEF files, which I did, and to their credit they did finally concur that there is an issue. I am now awaiting a prepaid shipping label to send the camera back.
What is very frustrating is that the camera was just there for two weeks for this exact problem - I wrote a detailed description of the issue and gave them both the links to my gallery showing the artifact as well as a link to the description by the Canon user. It seems that whomever serviced it during the first go around didn't actually investigate the problem by testing the camera at high shutter speeds - likely just ran the camera through some standard checks and stated 'no problem found'.