http://www.youtube.com/watch?v=qAHWBKpbvWk
That is a youtube video of my slightly outdated Sphere game. The main problem with that video is that the mass of the boxes was not computed. My new version has fixed this. The code is under the GPL however it isn't very object oriented, although it is rather cleverly engineered and I find it rather easy to read.
http://code.google.com/p/sjsayers93-jmegames/
Neat idea. how long did it take you and what are your plans for it?
It took me about 3 days since I was learning JME physics at the time, I plan on rewriting it in a game state and putting it in my new space game : http://code.google.com/p/tlc-spacegame/