Hmnn. Here we go again on pixel sizes... People keep forgetting that at the end of the day it's always the lens quality that matters, not really the pixels, especially when it comes to tele capabilities. You can always add tele-extenders (or for the astro-buffs eyepiece projection) to magnify the lens' image such that all its limitations become patently obvious. Let me explain. Any Canon supertele will have its job stretched quite beyond its "razor-sharp" capabilities if we use stacked 2x + 1.4x converters, on both the 10D or the 1Ds. Were this not the case then Canon would have offered us 3x or 4x extenders...
Well, I've not forgotten this.
. I think your bringing in other issues here; diffraction and the Rayleigh's limit.
I'm talking about ordinary (but expensive) telephoto lenses which are used at apertures well above the Rayleigh's limit, not astronomical telescopes
Of course, at the end of the day it's always the lens quality that matters. My comparison of the three cameras are in relation to the same
lens. If the lens is lousy enough, you probably won't notice much difference. If the lens is absolutely superb, the 1Ds might have the edge with a 2x converter. The Canon 1200mm, which costs as much as a house, might just trump the SD10 if used with a converter, but I'm not certain, and I don't expect anyone to show me some comparisons.