`multiprocessing` hangs, when `multiprocessing.Pool` methods `map`,
`imap`, `imap_unordered`, `apply`, `apply_async`, `map_async` are called.
Here is an example:

Python 3.0b1+ (py3k:64686M, Jul  3 2008, 13:06:13)
[GCC 4.2.3 (Ubuntu 4.2.3-2ubuntu7)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from multiprocessing import Pool
>>> Pool(processes=4).imap(lambda x: x, range(10))
<multiprocessing.pool.IMapIterator object at 0x83bcc8c>
>>> Pool(processes=4).map(lambda x: x, range(10))
Exception in thread Thread-13:
Traceback (most recent call last):
  File "/usr/local/lib/python3.0/threading.py", line 492, in
_bootstrap_inner
    self.run()
  File "/usr/local/lib/python3.0/threading.py", line 447, in run
    self._target(*self._args, **self._kwargs)
  File "/usr/local/lib/python3.0/multiprocessing/pool.py", line 225, in
_handle_tasks
    put(task)
  File "/usr/local/lib/python3.0/pickle.py", line 1319, in dumps
    Pickler(f, protocol).dump(obj)
  File "/usr/local/lib/python3.0/multiprocessing/util.py", line 311, in
_reduce_method
    if m.__self__ is None:
AttributeError: 'function' object has no attribute '__self__'

Process PoolWorker-28:
Traceback (most recent call last):
  File "/usr/local/lib/python3.0/multiprocessing/process.py", line 232,
in _bootstrap
    self.run()
  File "/usr/local/lib/python3.0/multiprocessing/process.py", line 88,
in run
    self._target(*self._args, **self._kwargs)
  File "/usr/local/lib/python3.0/multiprocessing/pool.py", line 57, in
worker
    task = get()
  File "/usr/local/lib/python3.0/multiprocessing/queues.py", line 337,
in get
Process PoolWorker-27:
Traceback (most recent call last):
  File "/usr/local/lib/python3.0/multiprocessing/process.py", line 232,
in _bootstrap
    self.run()
  File "/usr/local/lib/python3.0/multiprocessing/process.py", line 88,
in run
    self._target(*self._args, **self._kwargs)
  File "/usr/local/lib/python3.0/multiprocessing/pool.py", line 57, in
worker
    task = get()
  File "/usr/local/lib/python3.0/multiprocessing/queues.py", line 337,
in get
Process PoolWorker-26:
Traceback (most recent call last):
  File "/usr/local/lib/python3.0/multiprocessing/process.py", line 232,
in _bootstrap
Process PoolWorker-25:
Traceback (most recent call last):
  File "/usr/local/lib/python3.0/multiprocessing/process.py", line 232,
in _bootstrap
    self.run()
  File "/usr/local/lib/python3.0/multiprocessing/process.py", line 88,
in run
    self._target(*self._args, **self._kwargs)
  File "/usr/local/lib/python3.0/multiprocessing/pool.py", line 57, in
worker
    task = get()
  File "/usr/local/lib/python3.0/multiprocessing/queues.py", line 337,
in get
    self.run()
  File "/usr/local/lib/python3.0/multiprocessing/process.py", line 88,
in run
    self._target(*self._args, **self._kwargs)
  File "/usr/local/lib/python3.0/multiprocessing/pool.py", line 57, in
worker
    task = get()
  File "/usr/local/lib/python3.0/multiprocessing/queues.py", line 339,
in get
    return recv()
KeyboardInterrupt
    racquire()
KeyboardInterrupt
    racquire()
KeyboardInterrupt
    racquire()
KeyboardInterrupt
Terminated

The same happens on latest trunk of Python 2.6.
Note, `map` and `apply` methods hang Python interpreter completely, so I
have to use `kill` utility to exit Python.