Wouldn't they be able to find the power-loss when you unplug it, unless you create some sort of pass-through plug that you could use to plug the thing into the car occasionally to show use.
However, what happens if you get in an accident and the thing *isn't* in your car at the time. That could be a bit sticky, and in fact I had a friend run into the low-tech version of this. She was getting a discount for low-mileage on her car because she took the train to work, then decided the train was too much hassle and started driving downtown. Got into an accident and the insurance company started questioning the mileage the shop reported vs. the mileage they were estimating and she was self-reporting when they had her check-in. They came close to dropping her for that.
But back on-topic, I guess this is Progressive's attempt to compete with State Farm's deal with On-Star? Which I took advantage of with the Cobalt, BTW, since it is my low-use vehicle. I'll see what impact this has when I get my next premium bill, which, come to think of it, should be any day now.