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Russ, you're right about there being two behaviors for the 
implementation of the slice notation(see apply_slice() in 
Python/ceval.c).  This occurs in the bytecode interpreter 
rather than some object's implementation of __getitem__.

While it is unfortunate, it can't be fixed without breaking 
code which defines __getitem__ but doesn't provide a None-
to-zero converter.

Factiod of the day, interpreter doesn't just assume 0 for 
None, it clamps the values in a range of 0<= value <= 
sys.maxint:

>>> class C:
        def __getitem__(self, i):
            return i

>>> c = C()
>>> c[10000000000000000000L:]
slice(2147483647, 2147483647, None)


I recommend you close the bug as Won't Fix.