If we get back to my hastily penned comment which didn't express what I was attempting to say, namely;-
but I have no doubt that a 600/4 on a 1Ds2 would deliver higher resolution than a 400/2.8 on a Nikon D2X.
, the staement was supposed to be, I have no doubt that a 600/4 on a future full frame Canon camera with the same pixel densitiy as that of the D2X (ie. about 28MP total), would deliver higher resolution than a 400/2.8 on the Nikon D2X.
From the reviews I've read, the D2X almost has the same resolving power as the 1Ds2, but in the final analysis those extra pixels of the 1Ds2 do count for something.
Since I'm not a Nikon owner and have little knowledge of the relative merits of their lenses, I'm assuming they have lenses of equivalent type and quality to Canon's, so I'm ignoring the fact that in practice one would not be using a Canon IS lens on the D2X.
Bearing this in mind, I think it would be reasonable to state that a D2X with a lens of the quality of the Canon 400/2.8 would produce very similar results to the current 1Ds2 with Canon's 600/4. The slightly lower resolving capacity of the D2X would be compensated by the slightly higher resolving capacity of the 400/2.8, as compared with the 600/4.
But what's the advantage here, of the D2X/400mm combination. The two lenses weigh about the same (the 400/2.8 is just a few grams lighter). They cost about the same, (the 400/2.8 might be a few dollars cheaper depending on where you do your shopping). The DoF with the 400 at 2.8 is about the same as that of the 600 at f4 with the larger sensor.
There are two relatively modest advantages that I see,
(1) the 400/2.8 is a faster lens and should allow the use of a faster shutter speed with the D2X for the same DoF, FoV and resolution. But this advantage is eroded if the D2X has more noise (which it certainly does at higher ISO's).
(2) the D2X body costs less than the 1Ds2. However, anyone who is really into getting fine results from high quality lenses will find such cost savings relatively small compared to the total cost of their lenses.
If we now do a bit of crystal ball gazing, and predict a time in the near future when we have 30MP full frame 35mm cameras, then such a camera attached to a 600/4 lens should
according to my reckoning should produce a more detailed image than the current D2X with the 400/2.8.
Of course, in the meantime the D2X has not stood still. Perhaps we will by then have a 22MP D2X successor. My point is, there's a 'law of diminishing returns' at work here. At some level of pixel density, there's no more detail and resolution to be had without improving lenses. The pixels then just represent over-sampling to remove the need for an AA filter.
So I repeat, if the 'derogatory' cropped format is to survive, we will need a crop of DX and EF-S lenses that are off the Photodo scale of 1-5. There's going to be a squeezing from both ends. Whilst full frame comes down in price, P&S simultaneously goes up in quality. APS-C is not in an enviable position.