On Ubuntu 10.04, using freshly-compiled python-from-trunk (as well as multiprocessing-from-trunk), I get tracebacks from the following about 30% of the time:

"""
import multiprocessing, time           
def foo(x):                            
 time.sleep(3)                         
multiprocessing.Pool(1).apply(foo, [1])
"""

My tracebacks are of the form:
"""
Exception in thread Thread-1 (most likely raised during interpreter shutdown):
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/threading.py", line 530, in __bootstrap_inner
  File "/usr/local/lib/python2.7/threading.py", line 483, in run
  File "/usr/local/lib/python2.7/multiprocessing/pool.py", line 272, in _handle_workers
<type 'exceptions.TypeError'>: 'NoneType' object is not callable
"""

This information was originally posted to http://bugs.python.org/issue4106.