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 Mac OS/X 10.6
  "Snow Leopart" comes with a default
  Python that is 64-bit on 64-bit
  machines. The only way to use Psyco on
  OS/X 10.6 is by recompiling a custom
  Python in 32-bit mode.

In general, porting programs from 32 to 64 bits is only really an issue when the code assumes a certain size for a pointer type and other similarly small(ish) issues. Considering that Psyco isn't a whole lot of code (~32K lines of C + ~8K lines of Python), how hard could it be? Has anyone tried this and hit a wall? I haven't really had a chance to take a good look at the Psyco sources yet, so I'd really appreciate knowing if I'm wasting my time looking into this...
Psyco assumes that sizeof(int) == sizeof(void*) a bit all over the place. That's much harder than just writing down 64bit calling conventions and assembler. On the sidenote, pypy has 64bit jit support these days.
Cheers,
fijal
What are the possible pitfalls in porting Psyco to 64-bit?