Media Center is fully 32-bit.
The code is in good shape for a 64-bit compile, but this remains a couple versions out on the roadmap. Microsoft and Intel did a nice job of keeping great performance running 32-bit applications under a 64-bit environment, so we don't see a compelling reason for a native 64-bit version. It would be the same, just bigger and a little slower.
For what it's worth, I've been developing Media Center under Vista x64 for a while now.