I thought that Nikon didn't fab sensors or the surrounding microprocessing, only the body and component assembly.
I'm not talking about sensor fabrication, but the sensor assembly with the processing hardware, circuit boards, microlenses, AA filters, hot mirrors... The granularity of the process isn't the point, it's the modular re-use.
Modular common components is the design philosophy that underlies the products, which is why you'll see so many common sets of hardware in Nikon's product line.
The D3/D700 example shows how this works- same sensor components so "outselling" one or another becomes a pentaprism/flash/body choice more than it becomes a "which finished product?" choice. So does the D3/D3x, they're virtually identical outside of the sensor not for ergonomics, but for economics. Nikon only has to get and adjust the sensor component package and firmware and the machine that puts the camera name on to make one or the other- sell more D3 bodies, or more D3x bodies it essentially doesn't matter- the "cannibalizing sales" between models is only an issue if you don't have lots of common components. Nikon's built this strategy over the last several years and that's one reason why they can release "competing" cameras without so much worry about knocking their own sales.