Is the 180dpi based on the size of the print and the expected viewing distance? Ultimately what I'm trying to get a handle on is what is good enough for certain situations. This article has been cited on these forums before:
http://www.clarkvision.com/articles/...ppi/index.html
which has some really high numbers for what an eye can resolve, but I imagine that's under ideal circumstances that might not be required for large prints seen at longer distances. (I should really read the article all the way through; I've only skimmed a few times.)