Hi,
Aahrgh, sorry can't read my own code...The input pin in question is PortB.3 not PortA.0, sorry about that - and it IS set to an input.

No, the 18F2431 doesn't have any comparators and all pins are set to digital with ANSEL0 = 0.

Here's what the hardware looks like (again, that should read PortB.3, not A.0):
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As I've said, the hardware has been this way "for ever" and it has been working just fine "for ever". I changed the code and all of a sudden it didn't see the input. I could go back and forth between working and not working by basically changing where in the code I polled the input.

Again, the voltage at the pin WAS indeed outside the electrical specs but I still don't understand how it can see the input in one version of the code and not the other. The diode in series with the opto-isolator is NOW a shotky type and it's back up and running with the new code. I'm still looking for an explanation though.