With trunk when running test_multiprocessing in a tight loop I saw 
another problem:

test_multiprocessing
Process Process-61:
Traceback (most recent call last):
  File "/Users/cartman/Sources/py3k/Lib/multiprocessing/process.py", 
line 229, in _bootstrap
Process Process-60:
Traceback (most recent call last):
  File "/Users/cartman/Sources/py3k/Lib/multiprocessing/process.py", 
line 229, in _bootstrap
Process Process-62:
Traceback (most recent call last):
  File "/Users/cartman/Sources/py3k/Lib/multiprocessing/process.py", 
line 229, in _bootstrap
    util._run_after_forkers()
  File "/Users/cartman/Sources/py3k/Lib/multiprocessing/util.py", line 
138, in _run_after_forkers
    util._run_after_forkers()
  File "/Users/cartman/Sources/py3k/Lib/multiprocessing/util.py", line 
138, in _run_after_forkers
    util._run_after_forkers()
  File "/Users/cartman/Sources/py3k/Lib/multiprocessing/util.py", line 
138, in _run_after_forkers
    items = list(_afterfork_registry.items())
    items = list(_afterfork_registry.items())
  File "/Users/cartman/Sources/py3k/Lib/weakref.py", line 103, in items
  File "/Users/cartman/Sources/py3k/Lib/weakref.py", line 103, in items
    items = list(_afterfork_registry.items())
  File "/Users/cartman/Sources/py3k/Lib/weakref.py", line 103, in items
    for key, wr in self.data.items():
RuntimeError: dictionary changed size during iteration
    for key, wr in self.data.items():
RuntimeError: dictionary changed size during iteration
    for key, wr in self.data.items():
RuntimeError: dictionary changed size during iteration

The original problem itself seems to be fixed, so cheers for that!