Hm.  Well, after filing the bug, I created a thread for each subprocess,
and had that thread do an wait on the process, and that worked fine. 
So, I guess at minimum it sounds like the documentation for poll could
be improved to mention that it will not catch the state if something
else does.  I think a better fix would be for poll to return some kind
of UnknownError instead of None if the process was finished, but python
did not catch it for some reason (like using os.wait() :)