>I'm presuming professionals whose work demands they shoot high >numbers of exposures will wear out a pro Nikon in fewer years >than I will. (The D4's 400,000 shutter clicks will take me a >while.) That maybe why they replace so often.
Arthur, et. al.:
Yes, that's defnitely a big part of it. I know that if I were a full-time pro, I'd be having more frequent repairs done, and buying new bodies about twice as often.
Here's a great article by a working pro to put that into perspective: