if they release a 5dmk2 with an identical pixel density to that of the 40d, would agree there is no difference in what you are calling 'reach'?
For the most part, yes. I never said the smaller sensor was
why the 40D has more reach than my 5D. Again, my desire for a 40D is a practical one... for taking pictures of birds or other small and/or distant objects, the 40D is superior with the glass I currently own. Frankly, I have neither the desire nor the money to buy a 500/4L to attach to my 5D when I can get professional results with a 300/4L and a 40D.
would you also agree good glass wins over resolution?
Not sure exactly what you mean here. If you mean, "Does having longer glass on a 5D provide better images than having shorter glass on a 40D," then yes -- I agree. That's evident in the sample photos on page 2 of the thread I linked to. But what is equally evident in those photos is that the 20D provides more reach than the 5D when used with the same lens and focal length.
If you mean something else, then you'll have to clarify before I can say whether I agree or not.
there are quite a few canon (90-300, for example) and of course third party lenses that are very very poor at resolving fine detail. i'd say you'd definitely struggle to capture fine detail using lenses such as these.
The only way to know is to try it and see. Of course, it's quite unlikely that somebody trying to pick between a 5D and longer glass vs. a 40D and shorter glass would ever consider a lens like the 90-300. Almost all Canon prime lenses and many L-class zoom lenses easily outresolve all of Canon's DSLR sensors.
output magnification has a great deal to do with the appearance of bokeh as more of these areas will become visible the larger you print.
Yes, that's exactly what I said. But "bigger" due to greater output enlargement does not mean the
qualitative aspects of the bokeh are any different. The lens renders the bokeh to be smooth- or hard-edged, diffuse or distinct, etc. Your example (not quoted here) about taking a picture of a car with a medium format camera and then with a 35mm camera doesn't negate the point -- you're using a different lens on both cameras in 99% of those cases, and different lenses will render bokeh differently.
A bigger sensor means less enlargement to achieve the same output size, so of course OOF backgrounds look
different depending on sensor size. But bokeh, as I understand the term, does not generically mean "out of focus area." It's a particular
attribute of the OOF areas - the way OOF specular and non-specular highlights are rendered.
The more I think about this, the more convinced I am that we're just using the term 'bokeh' differently. There's no question that you get less DOF with larger sensors than smaller sensors. If that's what you're getting at, then I agree completely.
David