While we're on the topic of print resolution, I've heard lately that 150dpi (ppi?) is enough for most printers to get a very good without artifacts image printed on photo paper. So what's the standard these days?
I know my 1DS MKII will output a 300ppi image at 12x18, no crop. So you can see going any larger or cropping will need resampling to achieve higher ppi.
There are at least three issues:
1. What is the "native" grid of the printer. What pattern may be used for ink dots?
2. How is the signal flow from printing application to printer hardware. I.e. will the printer driver do something like bilinear interpolation or even nearest neighbor if you feed it a frame that does not coincide with pt. 1.
3. What is the acuity of our vision. In other words, at what point does further improvements not matter anymore.
I believe that you put too much weight to the "dpi" line of thinking, and the reluctance to do image resampling. I believe that the image _will_ be resampled at least once from camera sensor to print. Perhaps someone using specialized RIPs and not caring much about physical print size can prove me wrong, but it seems that the image pipeline is far too complicated and the user requirements so far removed from the "1:1 pixel" line of thought that it is nearly impossible to avoid image resampling. Any image is heavily procesed anyways (debayer is a kind of interpolation, dithering in the printer makes large changes to the image on a small scale). It may make more sense to inspect the end-result and see what kinds of artifacts and spatial resolution you are achieving. If the result is good, all is good, right?
For a given image, what kind of MTF figures can you hope to achieve with your 1DS (lense, focus, stand, sharpening/deconvolution...)? What is the "MTF" from your raw processor/printing application onto paper? Given these two, you may be able to:
a) Figure out how large you can print before the printer begins to significantly limit the actual end-to-end resolution
b) Figure out how large you can print before the prints start to look fuzzy to you at some given distance.