That's what the D700 actually was, so by substitution the D600 would be its replacement. But I'm not sure I think it is, because I would not classify it as prosumer. It lacks features pros would look for. The three kings are not as handy. The body is not magnesium. The shutter speed sync is a step backwards. The maximum frame rate is reduced. There is no AF-ON button. There is no similarity in controls.
I view it as being high end consumer. I bet its IQ might end up in a prosumer body, but not at a time that it would bite into D800 sales. I guess that might depend on D800 price drops. That's what I'll probably wait for.
The nice thing about 35mm film cameras was that they were all similar enough that you could pick any one up and operate it without retraining the brain. I thought Nikon had finally realized that and engineered it into their DSLRs, but I guess not. At least not, if you consider the D600 to be prosumer.
Scott Chapin Powder Springs, GA, USA Nikonians Team Member