The Psyco docs say:
Just for reference, Psyco does not work on any 64-bit systems at all. This fact is worth being noted again, now that the latest
+1 for "... how hard could it be?".
Take a look here: http://codespeak.net/svn/psyco/dist/c/i386/iprocessor.c
All that ASM would have to be ported, and there are assumptions all over the place about the underlying processor.
I think it's fair to say that if it were trivial to port (or not too difficult), it would already have been done.