Any ideas about converting the shocks into a usable signal (resistor divider) for input into Arduino? What is the typical shock voltage?
I don't have first hand experience with invisible fence dog collars, but I think it likely that in some (probably most) the detection circuit is separate from (i.e. both are not in an ASIC or FPGA IC because the shock voltage would be inconviniently high for that type of implementation) the electronics that cause the actual shock. In that case, you'd only have to tap into the wire or trace carrying the signal between the two circuits and either bypass or not worry about the shock.
BTW: Did I spell 'arduino' correctly, or does the 'arduino' forum editor not recognize the word 'arduino'?
The forum editor doesn't have a spell checking feature, at least not that I've been able to find. However, some web browsers do have built-in spell checking (I know the last couple of versions of Firefox do) and that's probably what is marking "arduino" as an unknown word.