The version of OSX that ships on the Retina MBP has specific software support for the retina display.
Even if you could manage to install Snow Leopard on it I think it would be a pretty bad experience. Everything on screen would be 1/4 the size it should be.
I thought that this might be the case. I guess we will try it to see how it turns out but i think a return to the apple store will come right around the corner, lol.
Does the regular macbook pro also ship with mountain lion?