I think, as a professional programmer, and having *LOOKED* at the tools microsoft provides for writing cross-platform games in the form of XNA...
That if you don't port your game to PC its because you don't want to, nothing more. Seriously, all you need to do is search replace "Gamepad.button.one" with "Keyboard.keys.one" virtually, the rest *should* more or less be done for you.
Also, if you code for ps3 and xbox you're already writing multiplatform code. Not porting to a third platform for a potential large number of sales is just plain dumb. There is *ONE* reason companies don't want their games on PC; PC gamers are used to the modding scene. Why buy Generic-FPS-3000 when you can just download a fan-made mod for Awesome-RPGFPSHybrid-2k and get 99% of the same experience for free?
Final thought: The 360 uses DirectX. MS Windows uses DirectX. Everything else is windowdressing and internal logic. IF you write your internal logic in such a way that a straight platform swap kills your game YOU ARE A BAD PROGRAMMER.
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