> Of course the latter isn't guaranteed to help for 
> non-IEEE-754 platforms -- some platforms don't have
> NaNs at all! 

ISTM, that years of toying with Infs and Nans has not
yielded a portable, workable solution.  I'm concerned
that further efforts will contort and slow-down our
code without benefit.  NaNs in particular are a really
difficult case because our equality testing routines
have a fast path where identity implies equality.  I don't
think NaNs can be correctly implemented without breaking
that fast path which appears throughout the code base
(even inside the code for dictobject.c; otherwise, how
could you lookup a NaN value).

I recommend punting on the subject of NaNs.  Attempting
to "fix" it will necessarily entail introducing a 
number of little atrocities that just aren't worth it.
IMO, Practicality beats purity on this one.

The decimal module provides full support for NaNs.
While it is slow, it may be sufficient for someone who
isn't happy with the way float handles them.