The Slow Cook has a post on a guy in Portland, Oregon, who got tired of waiting for a plot at a community garden, so he used Google maps to locate nearby empty city lots. Then he contacted owners about gardening on the vacant properties. One woman replied:
The risk, of course, is that in a few years he'll lose the land and all the work he put into it, but then I imagine there will be other plots for the taking.
Let every empty city lot be a garden! Imagine.