Check this out!
Somewhere in Brooklyn, will a full view of the Manhattan skyline, lies this amazing rooftop farm. The Eagle Street Rooftop Farm is a 6,000 sq/ft organic vegetable garden. The farm sells its produce and provides education programming for local kids and residents.
(photo from this beautiful the selby photoshoot)
Cincinnati has some great urban gardening, and a few productive urban farms–this one in Price Hill comes to mind–but none on a rooftop (as far as I know).
For an introduction to the woman that makes it all happen, go here for a nice video.