The multiprocessing package appears to spawn a new process by calling only fork(). Apple's CoreFoundation libraries (and possibly more, I do not know the full extent) *require* new processes to be spawned with the full fork()+exec*() combo.
When using PyObjC to access native Mac libraries, Python's multithreading library is not usable. The error thrown is:
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The process has forked and you cannot use this CoreFoundation functionality safely. You MUST exec().
Break on __THE_PROCESS_HAS_FORKED_AND_YOU_CANNOT_USE_THIS_COREFOUNDATION_FUNCTIONALITY___YOU_MUST_EXEC__() to debug.
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Test code: https://gist.github.com/1448398