SirNickity:
Well, a Zener is only going to clamp the voltage if it can bring the supply side down to its cutoff voltage. In other words, it will try to pull your 48v rail to 5v (or whatever). If it's a high-current rail, it will probably fail to do so, catastrophically. If it succeeds (a BIG Zener!), your 48v rail, isn't.The solution to this is to have a series resistance before the Zener. But then you have resistance (other than the current sensing "resistor" formed by the Rds of the FET) in your sensing circuit that you have to deal with.
If this is not news to you, you've found some suitable way to account for that series resistance, and you're able to to keep the shunt current within limits of the part, then I'm with the other guys here. The manufacturer should be able to supply reliability specs.
Thanks, SirNickity. You are quite correct I need a resistor before the zener! 
However, it has a negligible contribution to the "measurement circuit" because the zener is non-conducting (except for leakage) when I'm measuring. It's not until the voltage rises (the MOSFET turns off) that the zener starts to conduct and then there is a voltage drop on the resistor. (No current means no voltage drop).
Just to be sure I was correct in my thinking, I tested in multi-sim 
Thanks Jack! I figured they would be reasonably reliable, probably better than any relay I would put in there, but I don't have much practical experience with zeners for some reason...