It's like a giant Chia Pet: the 100-foot walls of the National Theatre in South Bank, London are covered in grass.
Which begs raises the question: how do you mow it?
http://www.thisislondon.co.uk/events/article-23395628-details/How%2Bdo%2Byou%2Bmow%2Byour%2Blawn/article.do
And I think goats would be better than cows. You could strap magnetic space boots on them, and magnetize the building.
Now that would be cool.