[Raymond]
> ...
> NaNs in particular are a really
> difficult case because our equality testing routines
> have a fast path where identity implies equality.

Works as intended in 2.5; this is Windows output:

1.#INF
>>> nan = inf - inf
>>> nan  # really is a NaN
-1.#IND
>>> nan is nan  # of course this is true
True
>>> nan == nan  # but not equal anyway
False