Roger, many years ago (10/78) the Modern Photography magazine that used to be published an article on the question "Is 100 lp/mm on film attainable?" Their answer was a highly qualified "Yes, under ideal conditions. In general, no."
The ideal conditions included: taking great pains to eliminate camera and subject movement; bracketing focus; using the highest resolution films available to them (High Contrast Copy developed for continuous tone; TP; SO 2483); still and clear air; and, oh yes, using a high contrast target.
They found that f/1.7 - f/2 "normal" lense for 35 mm SLRs could just do it under the ideal conditions sketched above, and typically only at f/4 or f/5.6.
Given the technique and emulsions most of us use, not to mention the conditions we work under, e.g., low constrast subjects, and the apertures we typically use, 100 lp/mm on film is fantasy. Every time I see Zeiss' results and the claims made for, e.g., Agfa Copex developed for continuous tone (=, I suspect, H&W Control, Gigabit film, Bluefire Police), I start doubting the sanity of the people who make the claims. I'm not convinced that the results, if true at all, are easily repeatable or that in practice we can come close to them.
There's another point about diffraction limits that many of us slide over. Since some of the major lens aberrations are off-axis and are somewhat controlled by aperture, the distance off-axis at which diffraction limits the resolution attainable (Strehl ratio > 0.85 is, I think, the conventional definition) is somewhat dependent on aperture. Brian Caldwell has made the point that the 55/2.8 MicroNikkor AIS is diffraction limited at f/4 in the central ~ 8 mm (that's diameter) of the image. Outside of that very sharp little circle, aberrations swamp diffraction. That, incidentally, is why its such a fine performer reversed at magnifications above 1:1.
Cheers,
Dan