I have made my decision. The initial platform will be Mac OS X (probably >= 10.6 for now). I am going to use MacRuby to develop this game. One of the reasons I want to do this is to be able to create a focused polished game. Even though I am going to use MacRuby, I would like to develop the game with the idea of porting it to other platforms later. This will lead me to separate the game logic and the like from the UI and platform details. If it makes sense, maybe later I could hook into another platform library.