The concept of "guerilla gardening" isn't that new. It's been around for years.
Basically the idea is that growing plants in abandoned areas of cities etc., helps to improve the quality of life for dwellers.
An Irish company is now selling cost-effective "seed bombs".
From their site:
"hand rolled from a mixture of clay, compost and Irish wild flower seeds with an average of 2500 seeds per Seed Bomb"
Full details here.
Update August 2015: Looks like the Irish site linked to above is no longer operational. If anyone knows of alternatives please let me know via the comments