Okay, x64 compilation on Windows is, in theory, possible. There are a few errors, but nothing that can't be fixed in the code.
The big issue is that, while compilation is possible, linking isn't. There are a few libraries like OpenAL, libogg, and the DirectInput stuff that refuse to cooperate. Everything we compile ourselves is, unsurprisingly, fine.
Of course, that's before we go into the thousands of compilation warnings regarding int, __int64, and size_t, which would require checking out.