It's still there, but masked by the creation of a new 'None': 
>>> None
>>> None = 1
>>> None
1
>>> import builtin
>>> builtin.None
>>> 
As Guido stated, in a module or procedure, this will create a 
new local None.
I uncovered my problem when I edited a module that assigned to
None, and inserted an assignment to another variable OF None 
before that assignment. The compiler then complained with a 
NameError: the use of None on the LHS told it to create a new
local variable, but the use on the RHS before it had a value 
was an error. ( without the assignment, it would have used the
Global value of None. ) 
I only continue to beat this dead horse into the ground because
it displays some of the peculiarities of scope. 
- Steve