Here's a 2009 video about the concept and execution.
In case anyone is interested, I saw a clip about the New York exhibit. After the pianos were located, cleaned up and painted, a piano tuner was hired to keep them in shape. Each piano had a volunteer "buddy" whose job it was to lock the keyboard every night and put a tarp over the piano to protect the instruments as much as possible under the adverse conditions. I'm sure the pianos still suffered (weather is the enemy to a piano), but I'm sure the art and instruments provided a lot of joy to the people who stopped by to play. There are videos showing singers and bands trying to travel around and play as many of them as possible (New York had 60 pianos). Quite a feat. And fun!