From this story on CNN, I learned about the Oxford Project.
In the spring and summer of 1984, Peter Feldstein used a red marker to make a sign announcing that he wanted to take free portraits of everyone in Oxford, Iowa (pop. 673)... Twenty-one years later, Peter set up his camera again. Some of the original residents had died and some had moved away, but a surprising number still lived in Oxford.
I'd love to check out the book, but, given my tendency to feel morose about time passing, I suspect it might not be a good idea.
Update: Thanks to photographer/author Peter Feldstein for stopping by in my comments and providing this more comprehensive link to The Oxford Project.