> This is wrong (the GIL must be held when using the PyMem_* and
> PyObject_* functions), and is probably the cause of the failed 
assertion.

This sounds quite likely.

I just managed (using the low-tech method of setting a static variable 
on entry and clearing it on exit) to confirm that PyObject_Malloc in 
obmalloc.c is being accessed simultaneously by multiple threads when
test_multiprocessing is run.