Resolution Doesn't Matter
Prepared 2008-01-19 (149/111777) by Bill Claff

 

I particular, photosite size is not a factor in noise or diffraction as it applies to our photography.

To understand the reasons you need a working knowledge of visual acuity and the Circle Of Confusion (COC).
The topic of COC is widely covered, particularly as it relates to Depth Of Field (DOF).
One place I recommend reading is Circle of Confusion under "Backgrounders" at my site.

Between 6 and 13 photosites are visually combined within the COC values we typically want.
The computation is N = ((COC / 2) / photosite)2 * pi
Because of both the Color Filter Array (CFA) arrangement and Nyquist frequency considerations we probably don't want N to fall below 4.
For a COC of .022mm N is 6.25 for the D70, 10.39 for the D200, and 12.54 for the D300.
For a COC of .033mm N is 11.98 for the D3.

You can rearrange the equation for N to solve for photosite:
photosite = ((COC / 2)2 * pi / N)1/2
By substituting your smallest desired COC and the value of 4 for N you can determine the largest photosite you can tolerate for your work.
For a COC of .022mm that's 9.75 microns.
For a COC of .033mm that's 14.6 microns.
Every current Nikon DSLR easily meets these standards.

You can also rearrange the equation to solve for COC:
COC = 2 * (photosite * pi / N)1/2
For N = 4 that's .018mm for the D70, .014mm for the D200, .012mm for the D300, and .019mm for the D3.

The grouping of photosites within the COC is the visual equivalent of "pixel binning".
You can "Google" "pixel bining" for further background. (For example Pixel Binning)
The important concept is that grouping photosites improves the Signal to Noise Ratio (SNR) and therefore makes noise less apparent.
This is why I argue that my concept of Photographic Dynamic Range as opposed to the "engineering" definition of Dynamic Range (DR) is what photographers should care about.

This grouping is the same reason that photosite size is not important in the discussion of diffraction.
It does not matter whether the Airy disc is larger than an individual photosite.
What matters is whether the Airy disc is larger than the group of photosites that comprise the COC.

Bottom line: become more aware of the COC value you need for your photography and stop worrying so much about photosite size (resolution) per se.