I'd searched around for this for ages myself and had presumed that we just weren't making the x64 bits available, but it turns out that it's been linked from the bottom of the download page all the time and I was just blind. If you're running Windows XP Professional x64 Edition then you can use the 64-bit release of the WinFX Runtime Components in combination with the regular 32-bit SDK and developer tools to get the full working developer environment.
Of course, since WPF applications themselves are managed, there's no need to distribute separate 32-bit and 64-bit editions of your own application, since the CLR JIT compiler takes care of converting the IL into the relevant processor instructions.
Download the x64 bits here; pick up the x86 bits and the SDK / tools from this location. (And thanks to Arik Cohen for pointing out what should have been right in front of my nose in the first place...)