The units I *have* found (all of them wired) demand to be on the last zone. If they're on the first zone, the water comes on and runs awhile, the sensor says "Ooh, nicely moist now", and shuts down the controller, never giving the other zones a chance to run.
Now why didn't I think of that? You're a good guy, blueman. I don't care what it says on the bathroom wall down at the truckstop. :-)
Interestingly, I got a call from a supplier the other day telling me that he has a unit that communicates through the valve wiring. I was driving, so I asked him to have his local dealer call my home phone. I haven't heard anything yet.