This past April I put 30 home made versions of an Earthbox on my flat garage roof.  I live in a relatively dense urban environment (Chicago), container gardening is the only way to go.  The Earthbox - http://www.earthbox.com/ - has a water reservoir, only needs fertilizing once, no weeding or mulching.  It's been quite an adventure; bugs, mold, mildew.......  

I'm trying to get the photos on a flicker page.  In the meantime, here's a couple links.

Homemade earthbox - http://www.josho.com/Earthbox.htm

Also, the City of Montreal has a rooftop gardening program - http://rooftopgardens.ca/en?PHPSESSID=ead547e5df9625f40e148fff6e36f8ef

Here's a good one in Chicago that I just found - http://www.urbanhabitatchicago.org/projects/true-nature-foods/