This sometimes happens on the buildbots:

test test_multiprocessing failed -- Traceback (most recent call last):
  File "D:\cygwin\home\db3l\buildarea\3.x.bolen-windows\build\lib\test\test_multiprocessing.py", line 1191, in test_pool_worker_lifetime
    self.assertNotIn(None, finalworkerpids)
AssertionError: None unexpectedly found in [1788, 3984, None]

e.g. http://www.python.org/dev/buildbot/all/builders/x86%20XP-4%203.x/builds/4334/steps/test/logs/stdio

Does this only happen on Cygwin buildbots ?
If yes, then it might simply be an issue with Cygwin's fork implementation, which is much slower than natively.
Right now, the test waits 0.5s before checking that the processes are started, after repopulating the pool. While 0.5s is normally way enough for forking a couple processes, it seems that under Cygwin this can take a surprising amount of time, see http://old.nabble.com/Slow-fork-issue---Win-x64-td19538601.html and also http://superuser.com/questions/133313/can-i-speed-up-cygwins-fork
Unless I misunderstand their benchmarks, the fork (+exec) rate of "date" from a shell can be as low as 5/sec, so I can only guess what forking cpython would take.
Maybe we could try to increase the timeout before checking the PIDs:

+    countdown = 10
-    countdown = 10
    while countdown and not all(w.is_alive() for w in p._pool):
        countdown -= 1
        time.sleep(DELTA)

There is no Cygwin buildbot, this is a regular Windows buildbot (despite the path in the traceback).
That said, spawning processes is quite slow under Windows anyway (much slower than under Linux). So we could up the countdown.

New changeset c4a514199dba by Antoine Pitrou in branch '3.2':
Issue #11766: increase countdown waiting for a pool of processes to start
http://hg.python.org/cpython/rev/c4a514199dba

New changeset 3eac8302a448 by Antoine Pitrou in branch 'default':
Issue #11766: increase countdown waiting for a pool of processes to start
http://hg.python.org/cpython/rev/3eac8302a448

New changeset 2e4cdaffe493 by Antoine Pitrou in branch '2.7':
Issue #11766: increase countdown waiting for a pool of processes to start
http://hg.python.org/cpython/rev/2e4cdaffe493

Hopefully fixed now, thanks for your diagnosis!