In reply to keith-ratcliffe:
Basically what you say is correct, the digital version has to be higher 'resolution' (I say that because the term is not so well defined for film) that the film so the digital version can accurately capture the texture of the film grain (be it from a scanned neg, print or artificial grain applied digitally).
I find it fascinating that film grain is so pleasing on the eye compared to digital artefacting. I think it must be because there is an intrinsically organic nature about the film grain. By adding it you actually create (to me at least) the illusion of more detail - your eye seems to accept the grain and see past it, interpolating the smaller detail. Whereas with digital artefacting it sticks out like a sore thumb and distracts from the image. Personally I don't routinely add grain digitally and I haven't used film for 12 years, but I can see the appeal.