The Center for Maths Science in Cambridge, England, has a grass roof over a central area: http://www.cms.cam.ac.uk/
the web site has a bit of information but not much. It is dramatic building, but not as well loved as somewhere to work as the nearby Isaac Newton Institute.
If anyone remembers, there was an episode of “Our House” a year or two ago with a couple building a house which had a grass roof. It was covered over a few episodes until the house was completed so it shouldn’t be too hard to get info on it.