This is a regression from 2.4.  This seems to always have been undefined behaviour.  It looks like it was the result of the compiler changes (code is the same in both versions, but co_consts is diff):

Python 2.4.4c1 (#2, Oct 11 2006, 20:00:03) 
>>> def r(n):
...   if n == -0.0: n = 0.0
...   return n
... 
>>> r.func_code.co_consts
(None, 0.0)

Python 2.6a0 (trunk, May 30 2007, 21:02:18) 
>>> def r(n):
...  if n == -0.0: n = 0.0
...  return n
... 
>>> r.func_code.co_consts
(None, -0.0)