Digital photographers have never stopped asking for more resolution. The 21-megapixel Canon EOS 1Ds Mark III has now reached a point where that need has largely been satisfied. Improving upon 21 megapixels means eliminating the sharpness-robbing anti-aliasing (AA) filter that both Canon and Nikon insist on using (an AA filter is either not present or is optional with medium format backs, one reason for their high resolving power). Eliminating color filter array interpolation would also result in a leap forward in image quality, but doing so remains an elusive goal. Finally, some responsibility remains with the photographer—absolutely spot-on focus and optimal aperture (f/5.6 - f/8) are required—nothing but perfect technique with top lenses will do.
Having examined the 21 megapixel images produced by the Canon 1DsM3, my conclusion is that lens performance is a limiting factor (including diffraction beyond f/5.6), but that there do exist high performance lenses which still have performance in reserve of and in excess of 21 megapixels, even across the entire frame. Such lenses include the Zeiss ZF 100/2 Makro-Planar, the Leica R 180/2.8 APO-Elmarit-R and certainly some Canon and Nikon optics as well.
Assuming the elimination of limiting factors like the AA filter, improved sensor technology, and evolution of lens technology, digital SLRs in the neighborhood of 36 megapixels might provide satisfying image quality. However, such a camera will require a world-class lens that must not be stopped down beyond f/5.6 (or maybe even f/4), making real improvements in detail elusive for practical photography. See for yourself in Diffraction—Canon EOS 1Ds Mark III.