I just found this project for an urban rooftop farm which is underway in Montreal near the Central Market, the equivalent of the Detroit Produce Market. The promoters built a huge greenhouse atop
an office building and soem links claim it is a first in North America. I doubt it, but what is interesting is they are looking for new buildings both in Montreal and cities in Canada and the US to develop their rooftop farms. Mind you, The density in Montreal warrants that kind of thing but Detroit is definitely on the right track if urban farming becomes the trend.

http://www.lufa.com/?l=en/