I'm thinking mine has failed now. I have 83,000 miles on the diesel and I've been noticing a persistent wet spot under the car for a couple of weeks now, and yesterday I received the low coolant warning.

I replaced my thermostat around 60k miles, as soon as I bought the car (which also resulted in needing to buy a new EGR cooler) and that's been fine, but no more. I haven't been able to actually get in there to look yet, but now I'm debating whether I want to deal with this project again or just bring it to the shop.