On Linux Ubuntu equipped with Python 2.6.1 I get this:

>>> os.kill(99999, 0)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
OSError: [Errno 3] No such process
>>> print os.kill(-1, 0)
None
>>>


IMHO the latter case should raise "No such process" for consistency or,
at least ValueError, but surely not returning None.