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this occurs for me running on Mac OSX Leopard. The equivalent code
using "processing" in python 2.5 works fine, which is how I found this
bug - my code hung when upgraded to 2.6. Basically initiating a
multiprocessing.Pool inside of multiprocessing.Process hangs the
application. Below is code which illustrates the issue on both py2.6
and py3.0:
 from multiprocessing import Pool, Process
 import sys
 def f(x):
 return x*x
 def go():
 pool = Pool(processes=4) 
 x = pool.map(f, [1, 2, 3, 4, 5, 6, 7])
 sys.stdout.write("result: %s\n" % x)
 def go2():
 x = map(f, [1,2,3,4,5,6,7])
 sys.stdout.write("result: %s\n" % x)
 # pool alone, fine
 go()
 # process alone, fine
 p = Process(target=go2)
 p.start()
 p.join()
 # use both, hangs
 p = Process(target=go)
 p.start()
 p.join()

Fails also on this system.
$ p3
Python 3.0.1 (r301:69556, Feb 13 2009, 23:52:55) 
[GCC 3.4.6 20060404 (Red Hat 3.4.6-3)] on linux2
(...)
>>> # use both hangs
(...)
>>> p.start()
>>> p.join()
(hmm la do diii laaa...)^C
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File
"/usr/users/lambert/bin/python/lib/python3.0/multiprocessing/process.py",
line 119, in join
 res = self._popen.wait(timeout)
 File
"/usr/users/lambert/bin/python/lib/python3.0/multiprocessing/forking.py",
line 118, in wait
 return self.poll(0)
 File
"/usr/users/lambert/bin/python/lib/python3.0/multiprocessing/forking.py",
line 107, in poll
 pid, sts = os.waitpid(self.pid, flag)
KeyboardInterrupt
>>>

Apparently the pool workers die all at once, just after the pool creates
them and before the pool is used.
I added a few lines to multiprocessing/pool.py to get the stack and the
exception backtrace.
 except (EOFError, IOError):
 import traceback
 debug(traceback.format_exc())
 debug(''.join(traceback.format_stack()))
 debug('worker got EOFError or IOError -- exiting')
 break
INFO::Rule dispatcher::multiprocessing::child process calling self.run()
DEBUG::Rule dispatcher::multiprocessing::created semlock with handle
3082559488
DEBUG::Rule dispatcher::multiprocessing::created semlock with handle
3082104832
DEBUG::Rule dispatcher::multiprocessing::created semlock with handle
3081826304
DEBUG::Rule dispatcher::multiprocessing::created semlock with handle
3081822208
INFO::PoolWorker-3:1::multiprocessing::child process calling self.run()
DEBUG::PoolWorker-3:1::multiprocessing::Traceback (most recent call last):
 File
"/home/who/.buildout/eggs/multiprocessing-2.6.1.1-py2.6-linux-i686.egg/multiprocessing/pool.py",
line 57, in worker
 task = get()
 File
"/home/who/.buildout/eggs/multiprocessing-2.6.1.1-py2.6-linux-i686.egg/multiprocessing/queues.py",
line 339, in get
 return recv()
IOError: [Errno 9] Bad file descriptor
DEBUG::PoolWorker-3:1::multiprocessing:: File
"/home/who/var/co/git-svn/what-base/correlator/bin/correlator", line 30,
in <module>
 what.corr.actors.main.main()
 File
"/home/who/var/co/git-svn/what-base/correlator/lib/what/corr/actors/main.py",
line 47, in main
 rrp.start()
 File
"/home/who/.buildout/eggs/multiprocessing-2.6.1.1-py2.6-linux-i686.egg/multiprocessing/process.py",
line 109, in start
 self._popen = Popen(self)
 File
"/home/who/.buildout/eggs/multiprocessing-2.6.1.1-py2.6-linux-i686.egg/multiprocessing/forking.py",
line 99, in __init__
 code = process_obj._bootstrap()
 File
"/home/who/.buildout/eggs/multiprocessing-2.6.1.1-py2.6-linux-i686.egg/multiprocessing/process.py",
line 236, in _bootstrap
 self.run()
 File
"/home/who/.buildout/eggs/multiprocessing-2.6.1.1-py2.6-linux-i686.egg/multiprocessing/process.py",
line 93, in run
 self._target(*self._args, **self._kwargs)
 File
"/home/who/var/co/git-svn/what-base/correlator/lib/what/corr/actors/rule_dispatcher.py",
line 26, in main
 initargs=(manager.agg_msgs_queue, ))
 File
"/home/who/.buildout/eggs/multiprocessing-2.6.1.1-py2.6-linux-i686.egg/multiprocessing/__init__.py",
line 232, in Pool
 return Pool(processes, initializer, initargs)
 File
"/home/who/.buildout/eggs/multiprocessing-2.6.1.1-py2.6-linux-i686.egg/multiprocessing/pool.py",
line 107, in __init__
 w.start()
 File
"/home/who/.buildout/eggs/multiprocessing-2.6.1.1-py2.6-linux-i686.egg/multiprocessing/process.py",
line 109, in start
 self._popen = Popen(self)
 File
"/home/who/.buildout/eggs/multiprocessing-2.6.1.1-py2.6-linux-i686.egg/multiprocessing/forking.py",
line 99, in __init__
 code = process_obj._bootstrap()
 File
"/home/who/.buildout/eggs/multiprocessing-2.6.1.1-py2.6-linux-i686.egg/multiprocessing/process.py",
line 236, in _bootstrap
 self.run()
 File
"/home/who/.buildout/eggs/multiprocessing-2.6.1.1-py2.6-linux-i686.egg/multiprocessing/process.py",
line 93, in run
 self._target(*self._args, **self._kwargs)
 File
"/home/who/.buildout/eggs/multiprocessing-2.6.1.1-py2.6-linux-i686.egg/multiprocessing/pool.py",
line 61, in worker
 debug(''.join(traceback.format_stack()))
DEBUG::PoolWorker-3:1::multiprocessing::worker got EOFError or IOError
-- exiting
INFO::PoolWorker-3:1::multiprocessing::process shutting down
DEBUG::PoolWorker-3:1::multiprocessing::running all "atexit" finalizers
with priority >= 0

It seems the root cause is at http://bugs.python.org/issue5155 .
A workaround is to use a duplex Pipe in SimpleQueue.

Patch attached to issue 5313, please review

Committed in r73708 on trunk