New York: 2008, 1978; 2009, 1609.

From The New York Times via DJ Bryan: Then/Now, a Series by David W. Dunlap

"The assignment in 1978 was simply to illustrate "The City Observed: New York," a guidebook to Manhattan by Paul Goldberger, who was then the architecture critic for The New York Times.

"My editors and I thought that pairing photos from then and now would be a graphic way to examine the phenomenon of urban churn that so defines this city. The series will visit a dozen or so neighborhoods, uptown and downtown, before the end of 2008. Each diptych tells its own tale, but the overall story is clear: It doesn't take much longer than a generation for New York to regenerate itself completely. "

It's well worth taking a look at this photo series. There is a cool flash application that allows you to use a slider to scrub back and forth between 2008 and 1978.

Also from the New York Times, this composite photos shows Manhattan Island in 2009 and a rendering of what it would have been like in 1609.