variable scoping across imports?

[Roy Mathew]

Dear Pythoneers,

I have what seems to be a vexing problem. 
In its simplest form, here are 2 files

# ------ file x.py ------------------------------------------------
z0 = None
import y

def set():
    global z0
    z0 = 33

if __name__=='__main__':
    y.show()
    set()
    y.show()


# ------ file y.py ------------------------------------------------
import x

def show():
    print x.z0

# -----------------------------------------------------------------
Now, I would expect that running "python x.py" would produce
  None
  33
but instead, it produces 
  None
  None

I have read the rules on variable scoping pretty carefully, I think.
Are the circular imports confusing python. (BTW, I am using 
"ActivePython 2.1, build 210 ActiveState").

What am I missing? 

Thanks,
Roy.