Ask HN: Graphics/Game programmers, How do you port 3d games across platforms?

I was discussing the topic recently, with regard to some of the Metal Gear Solid titles being ported to Xbox 360, and the Playstation Metal Gear Solid being ported to Gamecube, etc.

What does it take to port the game? Do new meshes have to be re-done? Does all the code have to be re-done? Or is it a 'simple' matter of changing a few files, tweaking Makefiles and building the codebase using the platform specific toolset?