This post is very old. Please bear in mind that information here might be incorrect or obsolete, and links can be broken. If something seems wrong, please feel free to comment or contact me and I'll update the post.
Since today, nivenCore works properly on x64, just tested, the unit-tests ran fine (tested on Windows Vista x64). Took me a while to recompile boost-1.33.1 for x64 (using the vc-8_0-amd64) toolset, but it was worth it 
Python 2.5 works, too, there is even a native x64 release on their page.
[Update] Added screenshot:
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