Russ, I usually shoot pretty wide open too, which is why I hadn't ever noticed such a problem. I had found an occasional spot, but that was it.
A couple of weeks ago during my one-on-one meeting with my mentor, he found out I was shooting with the D600 and asked if I had had any problems. I said no, and he said, "You probably shoot wide open all the time, don't you?" I hadn't even considered that as the reason I hadn't experienced any of the dust/oil problems that other D600 owners were experiencing. He advised me to stop it down and then check it out. You can see the results above!
I've got two upcoming assignments, and once I've finished those, I think I'm going to take the plunge and send the camera in to Nikon.