According the Environmental Protection Agency (http://www.epa.gov/hiri/strategies/greenroofs.html):

Green roofs are an attractive roofing option that can reduce urban heat islands by providing shade and, through evapotranspiration, releasing water from plants to the surrounding air. They also:
*Reduce sewage system loads by assimilating large amounts of rainwater.
* Absorb air pollution, collect airborne particulates, and store carbon.
* Protect underlying roof material by eliminating exposure to the sun’s ultraviolet (UV) radiation and extreme daily temperature fluctuations.
* Serve as living environments that provide habitats for birds and other small animals.
* Offer an attractive alternative to traditional roofs, addressing growing concerns about urban quality of life.
* Reduce noise transfer from the outdoors.
* Insulate a building from extreme temperatures, mainly by keeping the building interior cool in the summer.

And here’s some info from the American Society of Landscape Architects:

http://www.asla.org/land/050205/greenroofcentral.html

If you scroll down the page, there are some lovely pictures of their green roof here in DC.

I bought the most awesomest reusable bag there (the EZ Bag, that folds up and can fit in my pocket), a very cool Sigg bottle and plan to go and purchase a new thermostat this weekend. it’s a great store, the staff was helpful and they had a very nice dog sitting near the counter. the store isn’t brimming with merchandise, but what it does have is very cool and useful.