There are 3 sound sources, the mirror, the iris, and the shutter. All have to move, stop and be reset for an exposure. All of these items have some electronic control, but there is still physical movement of an object with mass.
Newer cameras have the silent mode - I.e. when you press the shutter the mirror lifts and the shutter fires, but the mirror does not drop (until you release the shutter). I think D3S, D5000 and D300S have it right now.
I am looking forward to trying this is certain situations that I shoot. I expect it will be there in all future enthusiast and above Nikon cameras.
D700 does not have a quiet mode but the D3S doe have the silent mode. We saw (heard) it at the Nikonians Mastering the D700/D3 workshop in Boston a couple of months ago. The class had a hard time discerning quiet mode from regular mode on the one D3S there...
Stop and take the picture, you may not get a second chance.