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On a Linux system (FC4) with r64686 of the Py3k branch I also still get
occassional hangs (with ./python -E -bb ./Lib/test/regrtest.py -v
test_multiprocessing). Mostly this seems to occur with the very first
test executed, i.e. before any of the "test_... " lines have been generated.
The following may or may not be related. Some time ago I decided to give
valgrind a try to see if it could detect anything strange going on with
the multiprocessing tests, specifically using the 'helgrind'
thread-debugging tool that comes with it. 
Valgrind reports as its first error:
==9719== Thread #1: Bug in libpthread: sem_wait succeeded on semaphore
without prior sem_post
==9719== at 0x4007FFF: sem_wait_WRK (hg_intercepts.c:1057)
==9719== by 0x4008094: sem_wait@* (hg_intercepts.c:1073)
==9719== by 0x46A0087: semlock_acquire (semaphore.c:310)
==9719== by 0x808C121: PyEval_EvalFrameEx (ceval.c:3371)
==9719== by 0x808D0FE: PyEval_EvalCodeEx (ceval.c:2808)
==9719== by 0x808B9D0: PyEval_EvalFrameEx (ceval.c:3469)
==9719== by 0x808D0FE: PyEval_EvalCodeEx (ceval.c:2808)
==9719== by 0x80F4B65: function_call (funcobject.c:628)
==9719== by 0x80D1207: PyObject_Call (abstract.c:2178)
==9719== by 0x80890EC: PyEval_EvalFrameEx (ceval.c:3672)
==9719== by 0x808C1A9: PyEval_EvalFrameEx (ceval.c:3459)
==9719== by 0x808C1A9: PyEval_EvalFrameEx (ceval.c:3459)
==9716== Thread #1 is the program's root thread
I've been hesitant to report this as the claim that libpthread is broken
is pretty bold. I contacted the valgrind devs about this, see [1]. 
More recently, someone on the valgrind list reported problems that do
seem to indicate there are broken libpthreads out there (see [2]), as
this individual reports a semaphore wait not blocking where it should.
Could it be that the multiprocessing tests are exposing one or more bugs
in libpthread?
[1] http://thread.gmane.org/gmane.comp.debugging.valgrind/8345
[2] http://thread.gmane.org/gmane.comp.debugging.valgrind/8384