Here's a site which has set a goal for having one million garden listed, as it's members on a social networking site.
The One Million Garden campaign is designed to identify, encourage, and document the creation of 1,000,000 food gardens throughout the United States by the end of 2009.
They have some fun videos
on the site too. Here's one showing how some folks used broken up concrete from a friend's garage.
Find more videos like this on One Million Gardens