When a manager receives a message, it unpickles the arguments; this
calls BaseProxy.__reduce__, which calls RebuildProxy.  If we are in the
manager, this returns the actual object, otherwise it returns a new
proxy.  If we naively disable the ability for proxied objects to be
unredirected in the manager, as in the attached svn diff, this solves
the problem that Carlos and I are seeing.  Surprisingly, after applying
this change, the full multiprocessing regression test still runs fine. 
I'm sure this change should have some greater impact, but I'm not sure
what.  I would appreciate if someone more knowledgeable could comment.