Okay, I'm not as dumb as I thought, I just couldn't believe you were relying on the zener's reverse leakage to pull the pin down. How would a resistor to ground in parallel with the zener affect the zener's capability to protect the input?
Edit:
Ahh, I see the source of some of my confusion, Andy's schematic is not a representation of the way your protection circuitry is connected. That explains everything. Thanks for the link : http://www.ruggedcircuits.com/html/mb324p.html#IOPins.