The New York Times has an article about a segment on digital cameras that was done for a television programme. The main point is that passers by were not able to tell the difference between the photos printed at different megapixel resolutions.
On the show, we did a test. We blew up a photograph to 16 x 24 inches at a professional photo lab. One print had 13-megapixel resolution; one had 8; the third had 5. Same exact photo, down-rezzed twice, all three printed at the same poster size. I wanted to hang them all on a wall in Times Square and challenge passersby to see if they could tell the difference.
In the comments section, differing opinions on the value of the experiment. One issue that I think about is what it would have been like to have shot the photos from cameras rather than using the same photo.