DVDdoug:
Contact/no-contact is "conceptually digital", not analog. (I understand what you're doing and I've seen it done before, but it kind-of bugs me when people use analog when they really want digital.)
There is no need for a voltage divider (unless you haven't told us everything) and there's no need for such a high resistance. Try a 1K pull-up resistor (with "contact" pulling down to ground). The lower resistance will probably solve your problems (and you probably won't need the over-voltage protection).
Yeah, analog read deeply bugs me too, but this is a real-world application in which the standard states something to the effect of:
- event 1: line 1 in contact with line 2, and over 100 Ohm with 3
- event 2: line 1 under 100 Ohm with 3, or 2 under 100 Ohm with 3
Then there are cases with 500 Ohm cut-off resistance...