PlixTixiplik said:
There was a recent non-fiction book called The World Without Us that describes the sequence of events if humans just disappeared. Scientific American had a summary article on the book, and an interview with its author, last month, and you can read it here. Some things would break down quicker than you might expect (the New York subways would flood within days), but other artifacts would last quite a long time (millions of years for some metal objects).
There's a link on that page to "The fall of New York" which has a nice timeline of approximately what would occur. Nuclear reactors would melt down in a about week as the cooling systems failed. In about 500 years New York would be covered by forests again.
-MEC
Interesting side-note. The author of this book was on The Daily Show last night.
For those in the U.S. interested in seeing it, there should be a repeat at 8:00 tonight.