well as this is my 100th posting, I guess i'd better make it a good one.
There is a point at which the human eye can not see any more detail, various things effect this, age, tiredness, how many beers you've had etc. However under good viewing conditions some things are just to small to see, so if the image contains these tiny details they will be to small for you to see. So for example if you take a 1Ds (11Mp digital), a EOS3 (35mm film) and a 645 of your choice and produce a 6x4 print you will not be able to see any more detail in one print than in any other. This is because there is more than enough information crammed into your 6x4 print so that all of the media that is supplying data are suppplying more than can be percieved at that scale. If you now do the same but take it to 12x8 then you will see that there is little difference between the 1ds and the 645 but the film based eos might be starting to show its limits. Get it up to 16x10 and the film is going to be showing its grain, whereas the 1ds and the 645 are still looking good. If you keep pushing it you get to a point where the 645 is going to have more data to play with than the 1ds, this seems to happen around 19x13 prints. Have a look at Reichmans review of the 1ds, he even does nice pictures.
LL 1Ds review
I guess it all depends on what the final output is going to be. It's not just about how big the enlargements are, its about sharpness, acutance, noise and grain. For prints under 19x13 the 1Ds has most other cameras beat.