NOTES
I owe this puzzle to Keith Lynch, the well-known local fan. Here's a link to a USENET thread in which it's discussed http://groups.google.com/group/rec.arts.sf.fandom/msg/2ae54a6d414c0dbb?d...
My first thought was that when the rock is released, there would be a recoil effect that would slow the planet's rotation, and move the planet to a slightly higher orbit as well; this does result in a decrease of total energy, while conserving angular momentum, but if this was that answer, that would suggest that releasing the rock in the opposite direction would lower the orbit, gaining energy. So that can't be right. After some further thought, I think I have a better answer.