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...
Since psyco is a compiler, it would need to be aware of the underlying assembly language to generate useful code.  That would mean it would need to know about the 8 new registers, new opcodes for 64 bit code, etc.  
Furthermore, to interop with the existing code, it would need to use the same calling conventions as 64 bit code.  The AMD-64 calling convention is similar to the old fast-call conventions in that some parameters are passed in registers (in the 64 bit case rcx,rdx,r8,r9 for pointers and Xmm0-Xmm3 for floating point) and the rest are pushed onto spill space on the stack.  Unlike x86, this extra space is usually allocated once for all of the possible calls.  The IA64 conventions and assembly language are different yet.
So in short, I think this is probably not as simple as it sounds.
What are the possible pitfalls in porting Psyco to 64-bit?