avr_fred:
This is because the Arduino pin protection diodes begin to conduct at approximately 0.3 volts...
Where did you get that info from.
If the datasheet says that you should stay under that limit, that doesn't mean it does already conduct at that point.
I measured those clamping points some time ago by by sending a 1mA current into a pin.
From memory, clamping voltages were something like 0.65-0.7volt.
So yes, an external schottky is wise, a common diode is not.
Leo..