Richard, can you say what failed on the OS X 10.4 (Tiger) buildbot?  FWIW, I tested b3620777f54c.diff (and commented out the darwin skip of test_multiprocessing_forkserver) on OS X 10.4, 10.5, and 10.8.  There were no failures on any of them. The only vaguely suspicious message when running with -v was:

./python -m test -v test_multiprocessing_forkserver
[...]
test_semaphore_tracker (test.test_multiprocessing_forkserver.TestSemaphoreTracker) ... [semaphore_tracker] '/mp18203-0': [Errno 22] Invalid argument
[semaphore_tracker] '/mp18203-1': successfully unlinked
ok
[...]
----------------------------------------------------------------------
Ran 233 tests in 97.162s

OK (skipped=5)  # on 32-bit 'largest assignable fd number is too small'
OK (skipped=4)  # on 64-bit

1 test OK.