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> do you think we should require that the world not
> change for 32-bit pickles?

I don't understand the question.  If a pre-2.5 pickle here
can be read in 2.5, where both producer & consumer are the
same 32-vs-64 bit choice; and a 2.5+ pickle here is portable
between 32- and 64- boxes, I'd say "good enough".

While desirable, it's not really critical that a 2.5 pickle
here be readable by an older Python.  While that's critical
for pickle in general, and critical too for
everyone-uses-'em types (ints, strings, lists, ...), when
fixing a bug in a specific rarely-used type's pickling
strategy some slop is OK.  IOW, it's just not worth heroic
efforts to hide all pain.  The docs should mention
incompatibilities, though.

Does that answer the question?