I made 70x100 print from my Sony Alpha 100, APS-C 10 MPixels. This was really intended as an experiment. The image was actually not optimal, because I needed to rotate it and crop. It also went trough some intensive sharpening. The print was than printed on a Lambda at 200 PPI (this is what the lab recommends for large sizes). The outcome was interesting. The image is obviously lacking in detail when looking at short distance, when I back off to about 80-100 cm the image turns pretty sharp and I see no further improvement backing off.
I don't have 20/20 eyesight and need correction glasses, so my findings would probably not apply to person thirty years younger, with 20/20 vision.
Another way to see it is that I often look at pictures as slide shows in 1080P (1920x1080) which is about two Megapixels (2MP!) on a screen 1.5 m wide, I need to sit like 2.5 or 3 meter from the screen, but it seems pretty good at that distance.
On the other hand, I also have a 100x50 cm panoramic stitch on my wall. This was made with the camera in vertical position and it was stitched from seven or nine pictures (with big overlap). Normally you see it at a couple of meters, because thinks standing in the way. If I walk up to that picture and look at very short range it is still critically sharp, but it is a mighty experience because I feel immersed into the picture. The picture essentially covers perhaps 150 degrees of view, so when I look at the picture at normal distance I have a false perspective, looking at the picture close by the perspective is much more real.
There used to be a rule of thumb that correct viewing distance would be focal length times magnification, in the panorama case correct viewing distance would be 50 cm.
A final observation is that sharpening can mask lack of detail as long as we don't have obvious halos. When looking at distance you cannot see detail anyway, but the brain says that because the edges are well defined the picture must be sharp.
So my conclusion is:
1) If you pixel peep all megapixels are needed
2) In many contexts you can do with much less especially if you can keep your viewers from pixel peeping
An observation on the side is that a lot of people I respect would say that it is easy to tell MFDBs apart from DSLR images also in small sizes. I would much like this phenomenon investigated and explained.
Someone on here quoted really good 7x-8x enlargements from scanning on the Coolscans. I have pushed a little bit beyond that comfortably - about 10x (so 10x15 inches from 35mm, and 20 inches from the 6cm dimension on 120 - 16x20 from 645, 20x20 from 6x6, as much as 20x30 from 6x9). How much beyond that (for a really good print of a high detail subject) will an Imacon go? A drum scanner? I can't imagine getting 20x, because that was widely considered impossible in the chemical darkroom (that would be a 20x30 from 35mm, and any print that size from 35mm I've ever seen was a grainy mess, no matter how it was produced). Is 15x realistic? A good darkroom worker could ALMOST do that, IF they started with a really good negative or chrome - that's where the special developers like Acufine and Microdol came in - people trying to make detailed 16x24 prints from 35mm. Of course there are low-detail subjects (or places where grain is acceptable) that print significantly larger, but that's true of digital as well.
The best digital sensors today will go something like 25x on the physical size of the sensor (I use the D3x as an example, because it's what I use, but I'm sure that there are other sensors that are in the same range). That gives us 24x36 from full-frame 35 (I do that all the time, and it looks great), 33x44 inches from the smaller medium-format systems and 36x48 from the P45+ size sensor. A P65+ with its oversize sensor SHOULD print right around 40x50, which is, coincidentally, also a 10x enlargement from 4x5.
There are a couple of problems with this... First is depth of field - I find myself REALLY thinking hard about DOF on big prints (using a bigger camera, one often has tilt to help deal with that - of course this is a problem with non-tilting medium format as well). My next lens purchase will certainly be a PC-E Nikkor (I'm trying to decide on focal length now). The second issue is when do you not actually need any more resolution for a bigger print, because of viewing distance? Ansel Adams raised this question in The Print in his chapter on very large prints. My 24x36's look good from a foot away - could I make a 40x60 that held up at 3 or 4 feet? If so, who'd ever look at a print that size from any closer than that? Prints that big are often displayed in ways where you can't GET closer than that...