Here is a nice trace on "PPC Leopard 3.x" thanks to faulthandler + regrtest timeout (30 minutes):
-------------------------------
...
[218/354] test_multiprocessing
Thread 0xf0617000:
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/threading.py", line 235 in wait
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/queue.py", line 185 in get
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/multiprocessing/pool.py", line 372 in _handle_results
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/threading.py", line 688 in run
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/threading.py", line 735 in _bootstrap_inner
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/threading.py", line 708 in _bootstrap

Thread 0xf0595000:
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/threading.py", line 235 in wait
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/queue.py", line 185 in get
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/multiprocessing/pool.py", line 331 in _handle_tasks
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/threading.py", line 688 in run
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/threading.py", line 735 in _bootstrap_inner
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/threading.py", line 708 in _bootstrap

Thread 0xf0513000:
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/multiprocessing/pool.py", line 324 in _handle_workers
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/threading.py", line 688 in run
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/threading.py", line 735 in _bootstrap_inner
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/threading.py", line 708 in _bootstrap

Thread 0xf0491000:
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/threading.py", line 235 in wait
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/queue.py", line 185 in get
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/multiprocessing/pool.py", line 102 in worker
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/threading.py", line 688 in run
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/threading.py", line 735 in _bootstrap_inner
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/threading.py", line 708 in _bootstrap

Thread 0xf040f000:
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/threading.py", line 235 in wait
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/queue.py", line 185 in get
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/multiprocessing/pool.py", line 102 in worker
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/threading.py", line 688 in run
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/threading.py", line 735 in _bootstrap_inner
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/threading.py", line 708 in _bootstrap

Thread 0xf038d000:
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/threading.py", line 235 in wait
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/queue.py", line 185 in get
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/multiprocessing/pool.py", line 102 in worker
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/threading.py", line 688 in run
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/threading.py", line 735 in _bootstrap_inner
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/threading.py", line 708 in _bootstrap

Thread 0xf030b000:
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/threading.py", line 235 in wait
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/queue.py", line 185 in get
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/multiprocessing/pool.py", line 102 in worker
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/threading.py", line 688 in run
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/threading.py", line 735 in _bootstrap_inner
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/threading.py", line 708 in _bootstrap

Thread 0xa09e1820:
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/multiprocessing/forking.py", line 134 in poll
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/multiprocessing/forking.py", line 149 in wait
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/multiprocessing/process.py", line 149 in join
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/multiprocessing/pool.py", line 458 in join
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/test/test_multiprocessing.py", line 1195 in test_pool_worker_lifetime
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/unittest/case.py", line 387 in _executeTestPart
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/unittest/case.py", line 442 in run
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/unittest/case.py", line 494 in __call__
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/unittest/suite.py", line 105 in run
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/unittest/suite.py", line 67 in __call__
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/unittest/suite.py", line 105 in run
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/unittest/suite.py", line 67 in __call__
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/unittest/suite.py", line 105 in run
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/unittest/suite.py", line 67 in __call__
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/test/support.py", line 1078 in run
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/test/support.py", line 1166 in _run_suite
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/test/support.py", line 1192 in run_unittest
  File "/Users/buildbot/buildarea/3.x.parc-leopard-1/build/Lib/test/test_multiprocessing.py", line 2141 in test_main
  File "./Lib/test/regrtest.py", line 1032 in runtest_inner
  File "./Lib/test/regrtest.py", line 826 in runtest
  File "./Lib/test/regrtest.py", line 650 in main
  File "./Lib/test/regrtest.py", line 1607 in <module>
make: *** [buildbottest] Error 1
-------------------------------
http://www.python.org/dev/buildbot/all/builders/PPC%20Leopard%203.x/builds/1634/steps/test/logs/stdio

 * The main thread (thread 0xa09e1820) is joining the pool (p.join()), wait in Popen.poll(): os.waitpid(self.pid, 0)
 * There are 4 workers waiting on their get() function, _Condition.wait(): if timeout is None: waiter.acquire()
 * The multiprocess Pool has 3 threads:

   * _handle_workers() (Thread 0xf0513000) is waiting in its polling loop:
       while pool._worker_handler._state == RUN and pool._state == RUN:
            pool._maintain_pool()
            time.sleep(0.1)
   * _handle_tasks() (thread 0xf0595000) is waiting on taskqueue.get()
   * _handle_results() (thread 0xf0617000) is waiting on get() method of another Queue

The main thread is waiting a child process, but we don't have the state of this process.