Is the 1Ds MkIII the successor to the 1D MkIII? The D3 competes with the 1D MkIII; the inevitable Nikon 24MP FX DSLR (which we have good indications of based on an accidental firmware upgrade released for the D3 that had JPEG quality settings based on 24MP files) will compete with the 1Ds MkIII and the Sony A900; it will not replace the D3, instead the "D3s" will replace the D3, but I'm fairly certain they will both use the same sensor.
I'm looking at this from the aspect of format size. Canon have doubled the pixel count of their full frame sensors over a period of approximately 6 years.
Nikon seem about to double the pixel count of their
FF sensors a year or so later than their first FF model was announced, perhaps a bit longer. I've got no idea if they will duplicate that (no doubt expensive upgrade) with a cheaper version like the D700, but it wouldn't surprise me if they were to.
I'm hoping that the Sony 24mp version will be the budget model because I already have a few Minolta lenses, but I don't have any Nikon lenses, so that's a problem for me.
The problem with resolution using these high pixel pitch DSLRs will not be the lenses, my Nikkor 14-24 does very nicely on my D300 and I'm sure it will do just as nicely on a high MP FX DSLR that has slightly larger pixel pitch (edge performance notwithstanding). Likewise, the Nikkor 24-70 has very impressive MTF numbers and I'm sure Nikon designed these lenses with a higher MP future in mind. The problem is that you will not see any improvements in resolution on a 24MP FX DSLR past f/11 over a 16 MP FX DSLR due to diffraction limitations.
As I understand, sensors with a higher pixel density but equal pixel count compared with a larger format, always
need a sharper lens in order to deliver the same image detail.
When maximum DoF is sought and one is shooting at apertures that begin to be noticeably affected by diffraction, it's not an issue. Any good lens at F16 with a 5D will deliver approximately equal total image resolution to an equivalent focal length lens on a 450D at F10. The 450D needs higher resolution from the lens, because of its greater pixel density, and it gets it, when used within the range of apertures affected by diffraction.
The problems begin when a wide aperture and shallow DoF are required for the composition. From the Canon perspective, you are then comparing (for example) a 450D with 50mm lens at F1.8, with a 5D and 85mm lens at F2.8. In these circumstances instead of the 450D getting the required higher resolution from the lens, that it needs, it's getting lower resolution.